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Why the Tricks Work

What Color Is Blue Ink?

As the filter paper absorbs the water-alcohol mixture, the mixture moves up the paper strip. When the mixture moves past the ink spot, the ink dissolves in the mixture and is carried along the strip. If the ink is a mixture of different colors, the colors will separate as they move along the paper strip.

Try this trick with different colored inks or with mixtures of food coloring. Have people predict what colors are mixed together and then check their predictions by this method.

The Invisible Force

Air is made up of tiny particles in constant motion. As these particles move, they exert a force, or pressure, on all surfaces they touch. Air exerts a pressure of 10 N/cm2 (14.7 lb/in2). This means air exerts a force of 10 newtons on every square centimeter of surface it touches. In this trick, the force of the air pushing up on one side of the card is greater than the force (weight) of the water pushing down on the other side of the card. So, the air pressure holds the card in place and keeps the water in the jar.

The Leakproof Strainer

After you’ve poured some water through a strainer, some of the holes in the strainer are covered with a thin film of water. This film is caused by the surface tension of the water. When you pour oil into the strainer, the wire mesh becomes coated with oil, even though the holes remain open. When water is poured into the strainer, the coating of oil increases the surface tension of water. The film of water that covers the holes is strong enough to keep the water from passing through the holes. So, the strainer fills up with water. If you touch the bottom of the strainer, you will break the surface tension and water will again pass through the holes.

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