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Martin Gardner's Science Magic: Tricks and Puzzles

Dover Publications, Inc.

Water

Cartesian Matches

An ancient toy called the Cartesian diver is a small hollow glass figure that moves up and down in a cylinder filled with water when pressure is applied or released to the air above the water. Here's an amusing way to demonstrate the same effect with two paper matches.

You'll need a plastic bottle with a tight-fitting cap. Fill the bottle to the brim with water; insert two paper matches, with heads down; and cap the bottle. If you now squeeze the sides of the bottle the pressure will force water into the fibers of the paper matches, causing them to sink. When you release the pressure, the matches rise. One match usually travels up and down ahead of the other. By adjusting the pressure, you can make each match move up and down as you please.

Magicians like to use matches with differently colored heads. Call one Mike and the other Ike. You can now make the matches respond to such commands as "Come up, Ike" and "Go down, Mike."

The Gorilla Effect

Before showing this trick, secretly rub your wet index fingertip on a bar of soap.

Fill a shallow dish, a saucer will do, with water and scatter black pepper over the surface. The water represents a lake. The pepper grains are bathers. Your finger, you explain, models another bather entering the lake. So saying, touch the tip of your middle finger to the water at the saucer's rim. Nothing happens to the "bathers." Repeat this a few times to represent other bathers entering the water.

Now, you continue, along comes a gorilla who has escaped from a nearby zoo. Again, your finger models the gorilla as he enters the lake. This time, however, place the tip of your soaped index finger on the "lake's" edge. Instantly all the "bathers" flee to the opposite side!

Somesault Shell

Carefully open a fresh egg so that the two half-shells are as similar as possible. Check to make sure that the shell for the larger end has an air bubble inside. Most eggs do.

If the bubble is there, you can mystify a friend with the following stunt. Fill a tall glass with water. Give the shell without the air bubble to the friend, while you keep the other half-shell. Say nothing about the bubble.

Put your half-shell, open-side up, on top of the water, and gently push on the shell's rim until the shell fills with water and submerges. As it sinks, the bubble will cause it to flip over and land convex-end up. Fish it out with a spoon and challenge your victim to duplicate the feat. When he tries, his shell stubbornly refuses to turn over.

Repeat a few times. After the last somersault, surreptitiously poke your finger into the shell to break the bubble. If your friend thinks your shell differs in some way from his, let him now try it with your shell. To his puzzlement, the shell still refuses to flip over.

Two 10-Cent Betchas

Adime is on the table beside a glass of water. The glass must have straight sides. Hand someone a straw and say, "Betcha can't pick up the dime with this straw and drop it into the glass."

Here's how you do it. Put a drop of water on the dime. With one end of the straw in your mouth, bend over so the other end of the straw presses vertically on the dime. When you draw in air, the dime will adhere to the straw, allowing you to carry it over to the glass and let it fall in.

Follow this with, "Betcha can't drop the dime several inches to the table so it lands and stands on its edge."

Secret: Dip the dime in the water and push it against the outside of the glass near the brim. When you let go, the dime adheres to the glass, slides down the side to the table, and remains on its edge.

A Water Transfer

Glasses A and B are completely filled with water. B rests on two table knives, which in turn are placed across the brim of C. The bet is that you can transfer all the water from A to C without touching the glasses.

The secret? With a straw, blow vigorously at the spot where the brims of A and B touch. The air blast forces water out of A, between the brims of A and B. The water will drip down the outside of B and into C.

But how, I hear you asking, are you able to fill A and B? Simple. Just put the two glasses below the surface of water in a sink, press the brims together, and lift out!

Water Level Riddle

Float a small glass in a beaker filled with water, then add to the glass marbles, pebbles, or other small heavy objects, until the glass is close to sinking. Mark the water level on the beaker. Remove the glass, dump the marbles into the water, and refloat the empty glass. Will the water level rise or fall?

Few students will guess that the level will fall. It seems plausible that putting the marbles into the beaker would make the level rise.

Two Corking Good Challenges

Challenges

1. Pour water into a glass until it is almost full, and then drop a small cork on the surface. The cork will drift to one side. How can you make the cork stay at the center?

2. Fill a beaker or pan with water and float a cork on the surface. How can you make the cork sink to the bottom without touching it?

Solutions

1. Carefully add water to the glass until the surface becomes convex, rising slightly above the rim. The cork will move to the center or highest spot.

2. Hold an empty glass upside down over the cork and push it down. Air trapped inside the glass forces the water aside, allowing the cork to sink.

The Dancing Triangle

Pour boiling water in a large bowl until it is about half an inch (1 cm) below the rim. Cover it with a cloth napkin or handkerchief. (Silk cloth works the best.) On top of the cloth put a triangle, cut from very thin paper, such as tracing paper, or paper used for second sheets in typing with carbon paper.

The paper triangle comes alive. It will curl up, then slowly uncurl and curl the other way, and keep up this curling and uncurling as long as the water remains hot. Its slow ballet dance may make it crawl over the edge of the bowl, but you can keep this from happening by constantly adjusting the cloth.

Try the experiment with a large shallow pan and a larger cloth. On top you can put papers cut into different shapes and watch them perform.

Why does this work? Moisture rising through the cloth expands the underside of the paper, causing it to curl; then the moist side exposed to the air dries out, the other side gets moist, and the paper curls the other way

The Marble and the Cork

Put a marble and a cork (or a table-tennis ball) inside a jar completely filled with water. If you put the closed jar on its side and spin it, what will happen to the two objects? Curiously, the marble will roll to an end of the jar, as expected, but the cork moves to the jar's center.

The centrifugal force field in the spinning bottle is directed from the center (at zero radius) to the ends (at large radius). Objects that sink in fluids in gravitational fields will also "sink" in the centrifugal field. Hence the marble sinks toward one end or the other of the jar. The cork will float "up" toward the region of zero centrifugal field (and up in the gravitational field).

A similar effect is displayed by a floating balloon whose string is held by someone in the back seat of a moving car. If the car accelerates forward, the balloon moves forward, not back. When the car slows down, the balloon floats backward. And if the car rounds a curve, the balloon moves toward the curve's center.

How to Measure Volume

Show your audience two small figurines that are different, for example, a dragon and an elephant. Which has the larger volume ? This may seem difficult to determine unless you think of this simple method. Plunge each into a beaker of water and see which figurine causes the water level to rise higher. Archimedes is said to have calculated the volume of a king's crown by this technique.

Three Jets

With a sharp pencil, punch three holes in the carton, as shown in the circle.

Keep three fingers against the holes; then fill the carton with water. Hold the carton over a sink and remove your fingers. The water will pour out in three streams, as shown on the left. The top stream will be weak, the middle one much stronger, and the bottom one the strongest.

Why it happens: The weight of the water causes a "pressure" in the water, which forces a stream out through each hole. The deeper the water, the greater the weight, and therefore the greater the pressure. The top stream comes from a shallow depth where the pressure is weak. The middle stream is stronger, and the bottom stream, of course, has the greatest pressure of all.

In lower depths of the ocean, the water pressure is so enormous that if deep-sea divers did not wear thick protective suits, it would kill them.

Air

Invisible Glue

Arrange five playing cards face down in a row on a cloth surface. Moisten your thumb and press it firmly on the center of card A. When you raise your hand, A sticks to the thumb. Put A squarely on B, all edges flush, push down hard, and lift. B will adhere. Continue to the right until all five cards have been picked up in this way and placed on the palm of your other hand.

Nob Yoshigahara of Tokyo, who invented this stunt, likes to pretend he is putting a drop of invisible glue on the middle of each card before he does the pickups.

Air pressure keeps the cards together. The trick works only on a cloth surface or hard rug; otherwise the cards stick to the surface.

Three for Bernoulli

Bernoulli's principle states that air in rapid motion will lower the air pressure along its path. Here are three easy ways to demonstrate this.

1. Hold a drinking glass in one hand and with the other hand hold a burning match behind it. Blow toward the glass. The flame goes out as if you blew right through the glass.

2. Cut two long narrow strips from a newspaper. Bend your head forward and hold the ends of the strips on either side of your mouth. Blow down between the strips. You will see their lower ends come together instead of moving apart.

3. Push a thumbtack or pin through the center of a business card. Holding the card horizontally, place a spool on the card over the tack. As you blow down through the hole, let go of the card. You would expect the card to be blown off the spool, but instead it clings to the spool's underside.

A Blow for Bernoulli

There are many well-known ways to demonstrate the Bernoulli effect, but here's a simple demonstration that is not so well known.

At one end of a ruler, fasten a paper strip with a slight hump in the center as shown. The strip's ends can be pasted down or held with rubber bands. Place the ruler crosswise on a round-stemmed pencil. Roll the pencil back and forth until the end of the ruler with the paper strip very slightly overbalances the ruler's other end.

Blow toward the ruler. Air rushing over the strip's bulge will lower air pressure above the bulge and cause the ruler to tip the other way

The Mysterious Balloon

Can a toy balloon be inflated and remain this way even though its mouth stays open? Here's how to do it.

Put a few inches of water in a large glass bottle that has a small opening. Set the bottle on a low stove light. When the water boils, remove the bottle and very quickly attach the uninflated balloon's mouth to the bottle's neck. The air inside the bottle contracts while it cools, drawing the balloon inside, turning it inside out, and inflating it. Now you have a curiosity to show friends. An inflated balloon with an open end! If the balloon has printing on the outside, turn it inside out before you attach it to the bottle. The inflated balloon will then have its printing on the outside, and look even more puzzling.

There are two unusual variations of this trick that do not require heating. Obtain a large, hard-plastic bottle of the sort soft drinks now come in. Punch a hole near the bottom. You can now insert an uninflated balloon into the bottle, attach its mouth to the bottle's mouth, then inflate the balloon inside the bottle by blowing into it. The hole allows the air to escape as the balloon inflates. After the balloon is inflated, a thumb over the hole keeps the balloon from deflating. Exhibit this inflated balloon, with its open mouth, as a curiosity. Wave your hand over it, pronounce a magic word, and then take your thumb off the hole. The balloon instantly turns inside out to inflate again above the bottle.

A second variation, using the same hard-plastic bottle, is to attach the balloon to the mouth as before, with a straw alongside it. The straw allows air to leave the bottle as you blow to inflate the balloon. While the balloon is inflated, remove the straw, leaving the inflated balloon permanently inside the bottle.

The Unbreakable Balloon

The elastic skin of a balloon is much thicker close to its neck, and also at the end opposite the neck than it is at other spots. A needle or a hatpin can be pushed into either of these spots without bursting the balloon. The holes are so small that the balloon seems to remain the same size after the needle is removed from the holes. Of course the balloon bursts if you stick the needle in at other spots.

An effective magic trick uses a balloon that inflates to a sausage form. Push it into a cardboard tube such as one that comes inside toilet paper rolls. As you push the balloon through the tube, secretly give it a twist. You now can push hatpins through the tube's middle as shown, taking care not to go directly through the balloon's twisted portion. For people who do not know that the balloon has been twisted, its failure to burst is mystifying. Withdraw the hatpins and secretly untwist the balloon as you remove it undamaged from the tube.

Fire

Candle Seesaw

I burn my candle at both ends.
It gives a lovely light.
Up and doum, up and down,
It seesaws through the night.

Cut away the tallow at the bottom end of a cylindrical candle to expose the wick. Push a needle through the candle's exact center. Rest the needle's ends on the rims of two glasses as shown, and put a plate under each end of the candle to catch wax drippings.

Light the candle at both ends. Give one end a slight push and the candle will start rocking up and down like a seesaw. The motion continues as long as the ends burn.

You might suppose that each time a candle end leaves a bit of wax on a plate it makes that end a trifle lighter, and that these changes in weight operate the little machine. But is this the case? Are the weight differences sufficient? Is there a way that the formation and dropping of the tallow can deliver a slight upward recoil? To find out you must burn your candle at both ends.

Miniature Rocket

Place a toothpick alongside a paper match. Under the top two-thirds of the match put a piece of tinfoil about an inch by two inches, as shown in Figure 1. Fold the rectangular foil over the match, along dotted line A; then wrap the foil firmly around the match. Withdraw the toothpick. This leaves a tiny open channel running to the match's head.

Bend an empty match folder in the middle (Figure 2) so it can serve as a launching pad. Light another match and hold the flame beneath the foil. When the matchhead ignites, gas escaping through the channel will propel the wrapped match six or more feet across the room.

Light from the Wrong End

Light a paper match and hold it vertically in your right hand. Hold an unlit paper match horizontally in your left hand as shown. You must be in a room where the air is totally free of drafts.

Bring the burning match over to the unlit one, and touch the flame to the head of the unlit match to light it. Now move your right hand away a foot or so, lean forward and blow out the match in your left hand. Bring the burning match back to the match you just extinguished, and touch the bottom of the burning match to the unlit head. It relights! Throughout these moves, your left hand must not move. Only the right hand moves away and then back.

Why does it work? There is a combustible component in the wisp of smoke that rises from the blown-out match. The flame jumps down this wisp so rapidly you can't see it. It looks exactly as if the bottom of the burning match relights the other one.

Heat

A Penny for Your Thoughts

Place ten pennies in a row on the table. While you go out of the room (or turn your back) someone is asked to select a penny, press it against her forehead with the tip of a forefinger, and hold it there while she counts slowly to fifty. The penny is then replaced in the row.

Explain that thought vibrations from her brain will be transmitted to the penny.

Return to the room. Touch a fingertip to each penny to detect its paranormal "vibrations." Sure enough, you correctly identify the penny she chose.

How do you do it? The selected penny will be distinctly warmer than the others.

Psychic Motor?

Stick a pin through a piece of cardboard. Fold a 2-inch square of paper in half both ways; then balance it on the pin's point as shown.

Hold your cupped hands on both sides of the tiny "motor." In your mind order the paper to rotate. Be patient. The chances are high that in a minute or two the little square will start turning. Try commanding it to stop and go the other way

Psychokinesis? No. It is almost impossible for a room, even with windows closed, not to have convection currents arise from temperature variations in the air. The heat of your hands, even the heat of your body, can produce such currents. Breathing may also play a role.

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Excerpted from Martin Gardner's Science Magic: Tricks and Puzzles by Martin Gardner, Tom Jorgenson. Copyright © 1997 Martin Gardner. Excerpted by permission of Dover Publications, Inc..
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