What you need
- empty plastic bottle with a lid
- pin
- sticky tape
- water
What to do
- With the lid on the bottle, use the pin to make a series of small holes in a line around the bottle.
- Wrap a piece of sticky tape around the bottle so it is covering all of the holes.
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Take the lid off the bottle, carefully fill the bottle to the top with water, and replace the lid.
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Being careful to avoid squeezing the bottle, peel off the sticky tape.
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Hand the bottle of water to a friend and watch what happens when they unscrew the lid!
What’s happening?
When the lid is closed, there are a number of forces acting on the water in the bottle. There are cohesive forces between the water molecules leading to surface tension which makes the water less likely to spill out of the small holes; there is the force of gravity pulling down on the water; and force from the sides of the bottle pushing in on the water. However, the main force that makes the trick possible is air pressure.
Although we don’t notice it, the air in the atmosphere is pushing on us, and everything around us, all the time. Air pressure pushes in on the bottle, and in on the small holes, preventing the water from falling down due to gravity.
When the lid is removed, air pressure also pushes in through the neck of the bottle and the force of gravity takes over, causing the water to stream out and onto your poor, unsuspecting friend.
