Here’s a classic activity with a showy twist. Surprise your friends by inverting a water bottle without spilling its contents!
You will need
- Piece of light flyscreen
- Scissors
- Bottle*
- Rubber band
* Hint: Smaller bottles with a narrow neck are easier to handle
What to do
- With an adult’s help, test whether your scissors can cut the flyscreen. To make cutting easier, keep the scissor blades apart. Use the first centimetre or so of the blades that are closest to the handles. If the flyscreen is hard to cut, stop. It’s unsafe to force scissors to cut tough flyscreen. Try a lighter flyscreen combined with strong scissors.
Carefully cut out a circle of flyscreen about 10 centimetres across.
Put the flyscreen over the bottle’s mouth. Gently bend the flyscreen down the neck of the bottle.
Put a rubber band around the bottle’s neck to hold the flyscreen in place.
Put the bottle under a tap and fill it up with water.
Time to play! Try pouring some water out of the bottle and into the sink. When you want to stop the water flowing out, keep tipping until the bottle is completely upside down.- Practice this trick a few times. Then you can shock your friends and family next time you pour them a glass of water!
What’s happening?
When water pours out of a bottle, air comes in! If one of these stops, the other will too.
When the bottle is completely upside down, the water stops pouring. Water tends to stick to itself, and this property is known as surface tension. The air can’t stretch the surface of the water from a tiny flyscreen square to a huge bubble. The water molecules don’t stick together particularly strongly, but it’s enough to keep air out and water in.
You can see how weak the force between water molecules is by tipping the bottle sideways. Suddenly, air can get in, even if the bottle is completely full! The trick here is a small difference in pressure.
When the bottle is on an angle, the water at the bottom of the mouth is being squeezed by all the water on top of it. This small pressure difference between the bottom and the top is enough to overcome surface tension, letting air into the bottle. The water then flows freely out of the bottle!
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