You will need: Two glasses, fizzy water (this also works with lemonade or any other see-through fizzy drink, but it is better with fizzy water as its usually more fizzy), plain water, two see-through cups, a few raisins (10ish).
Place the two cups on a flat surface. Into one, pour the fizzy water. Into the other, pour the plain (still) water.
Pop an equal amount of raisins into the each cup (good maths language, understanding the phrase equal!) and watch what happens.
Ask your child what the difference was in the two experiments. Why do they think this is? Note: The bubbles collect on the raisins and make them bob up and down in the fizzy water and they don't move in the still water.