All you need for this experiment is a clear bowl, a candle, water and matches.
Stick the candle to the bottom of the bowl, by putting some hot wax. Let it dry and make sure it is holding well.
Fill the bowl with water.
Light the candle and wait.
The candle will keep burning even under the water level, but then the wax will melt and water will go in. The candle will obviously stop burning.
It's a very easy, but fun experiment where you literally need nothing special.
Will keep the kids busy for sure, coz it takes ages for the candle to melt and get shorter!