First, the girls each put 1 tablespoon of baking soda on a small piece of paper towel and folded it up.
Then we put a half cup of white vinegar and a fourth cup of warm water in the bag (the water wasn't that warm by the time it finally made it to the bag). We sealed the bag, backed up, and waited.
The bags got really big, but they never actually popped...until E(2) stepped on them! The baking soda was old and the water wasn't very warm, which probably made the reaction less strong than it should have been. That was ok though, we still had fun!