General picture of science equipment

Sticky cups

Use air pressure to make two cups stick together. This experiment creates a vacuum.

Ingredients and equipment that you will need

Two rigid plastic (NOT glass) cups
Hot water
Kitchen roll

Method and further information

Put very hot water in each and leave them to warm up.

Cut a piece of kitchen paper so that it is just a bit bigger than the open end of the cup. Wet the paper in the hot water.

Now tip the water out of both cups. Put the wet kitchen paper on top of one and turn the other upside down on top.

Leave them to cool down. You should find that when you lift the top one, it is stuck to the one at the bottom.

Hot air expands taking up less space, so when you push the two cups together there is less air inside the cups than outside. When it cools down, the air pressure inside goes down. There is now a partial vacuum inside the cups. The higher air pressure outside holds the cups together.

Be careful – you should ask an adult to help you with these experiments. Do your experiments in old yoghurt pots, jam jars or bottle tops. Don’t use cups or glasses in case someone thinks it’s a drink.