First, she filled a water bottle 3/4 full with lake water. Then we built a sand volcano up around it. She put blue food coloring in the water (blue is her favorite color).
Then she put 2 tablespoons of baking soda in the bottle.
I poured in the vinegar, and we watched our volcano explode!
We made it explode several times before O started experimenting with putting sand in the bottle, etc. This was such a fun activity!