You can take a small water bottle make a base around the bottom with play dough and build it up around the bottle till you come to the top. Decorate it however you choose. Then put baking soda into the bottle and add vinegar. It does make a mess, so have some towels handy. It is fun experiment and your kids will enjoy it. My little five year old girl wanted me to keep pouring in the vinegar, so she could watch it spew out.