My Paper Mache Volcano

This is a paper mache volcano and it's solid. This is how I made it:
I started off by crushing the newspaper and we dipped the paper in glue. Then we let it dry and it went hard.



I made it explode. I put vinegar and red paint in the volcano and then mixed in some soda powder. It exploded when we mixed it together.

Hmm - I wonder what made it 'explode'. How do you think we can find out?