Popcorn rocks were neat to watch but took a few weeks to grow. We got a bag of these special rocks at a Science/History museum. It took about 3 weeks in a bowl of vinegar to grow "popcorn". It was neat to watch the rocks fizz and bubble the first few days. The package said that the popcorn would harden with time but it was to soft after sitting for almost 5 weeks for the kids to keep.