Fun Science: Blow Up a Balloon Without Blowing It

This one definitely needs adult help and make sure you discard the liquid straight after to avoid any accidental drinking of it. I loved this experiment, it appealed to my Microbiologist background (without the need to grow mouldy stuff lol). My son thought I was tricking him when we did this and he still doesn’t truely understand what happened.

What you will need:

Dried yeast (you can find it in the baking aisle of the supermarket)

Warm water (tap temperature, not boiling water or you will kill off your yeast)

1 teaspoon of sugar (the yeast’s food)

Plastic bottle

Balloon (any colour!)


Pour the warm water into the bottle and add the sugar and the yeast.

Put the balloon over the mouth of the bottle and give the water and yeast mixture a gentle swirl.

Put the bottle on a sunny windowsill and wait.


Ask your child what has happened to the balloon. Can they explain it?

Cheat sheet:

Yeast is a living thing. By adding the sugar and water to it, we let it grow, and as it grows it produces a gas, carbon dioxide. Eventually enough gas is produced that it rises into the ballon making it grow in size as well.

Top tip! Use a small bottle or it takes ages. Something like a mini pop bottle. Don’t use a big two litre bottle you will be waiting for ages. Keep an eye on it, if your yeast are warm enough and grow quickly the balloon grows in size quickly. You don’t want it to pop and the bottle to fall over!


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