Baking soda and vinegar

Put a tablespoon of baking soda in each well of a muffin tin (or a single small dish). Put vinegar in a small cup with a dropper. Place on a tray.

Drop one drop of vinegar onto the baking soda. Watch the fizz. Drop another.

Astonishment, every single time, even on the tenth play-through. "It's bubbling!" Most kids will dropper-by-dropper across all the wells, then ask for more vinegar.

Add a drop of food coloring to each well so the reaction is colorful. Replace dropper with a small spoon for less control. Try lemon juice instead.