"Exploding" hot chocolate

My mug was broken into pieces so I made hot chocolate in a container. I lightly sealed it with a cover. I put it on a table to let the temperature cools cool down. About 30 seconds later, the cover was thrown away flew off with high speed due to the great amount of pressure produced by the hot steam. My friend and I were taken aback because we thought there was a bomb! Wow! We made a "science experiment" without intention intending to.

Wow, cool story!