They're quick and easy but the results are eye-opening!
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A Dry Dip Into Water
Place your arm in a plastic bag. Make a fist and then submerge your arm into at least 12 inches of water—the deeper the better. Try opening your submerged fist. It's very difficult!
What's Happening:
Water has weight. It's this weight that pushes against the plastic bag, which pushes against your arm. The deeper you go, the greater the push. Because water is heavy, you don't need to go very deep before the push is strong enough to prevent you from opening your fist. This explains why snorkels have to be short. Long snorkels allowing you to swim deeper don't work because our lungs are not strong enough to expand against the great weight of water.
The Invisible Push
Fill a bottle with water. Place a card on top of the bottle. Holding the card in place, turn the bottle upside down. While holding the upside-down bottle, ask your child to predict what will happen when you let go of the card.
What's Happening:
Air has weight. The weight of this air pushes against us in all directions. The force of this push at sea level is about 14 pounds for every square inch. So how does the card hold up the water in the bottle? The downward push from the weight of the water in the bottle is less than a pound. The upward push from the weight of the air, however, is about 14 pounds. The air wins! Although invisible, air is real stuff. Because of this, birds and airplanes are able to fly.
Weightless Water
Poke a small hole in the side of an aluminum can toward the bottom of the can. Hold your finger over the hole and fill the can with water. Ask your child what will happen when you remove your finger from the hole. Of course, water comes out in a stream. Ask your child if the water would still come out if you were to drop the can. Try it and see. To avoid a mess, drop the can into a bucket. Note carefully that as the can falls, the water stays in!
What's Happening:
As you hold up the can, the water flows out through the hole because of gravity, which is a force that pulls things downward. When you release the can, gravity continues to do the same thing—it pulls the water downward, only this time it pulls the can down with it too. Because the can and the water are falling together, there is no reason for the water to flow out of the can. For advanced thinkers: As the can falls and the water doesn't come out, it appears as though gravity has turned off. Indeed, as anything falls, there is this sense of "weightlessness." You have this weightlessness when you jump off a diving board. Interestingly, astronauts in an orbiting space station are NOT free from gravity. Instead they are falling. The only reason they don't hit the ground is because they are also moving sideways (at 17,500 mph). Hence, they are falling "around" the Earth, rather than into it. They experience exactly what you experience off the diving board, except for a much longer time.
Circular Rainbows
Place a concentrated dot of ink from a black felt-tip pen at the center of porous paper, such as a napkin or coffee filter. Wet your finger with water so that there is a drop of water at the tip of your finger. Place this drop on top of the dot and watch the ink spread radially with the water. Just after the drop is completely absorbed, add additional drops until the ink has separated into a beautiful circular rainbow.
What's Happening:
Black ink is made by mixing together inks of different colors. Some of these inks are strongly attracted to the paper while others are weakly attracted to the paper. If the affinity is strong, then the ink prefers to stay bound to the paper rather than traveling with the water. If the affinity is weak, the ink prefers to travel with the water. As the water migrates through the paper, the different colors of ink are separated from each other because of their differing affinities for the paper. Experiment with different pens; some work better than others. Secretly write a few words with one of several pens and have your child do an experiment to find out which pen you used.