Surprisingly enough, a few days later when we went to the farmer's market we found this book:
The book was a little wordy, but in the back there was a little experiment about creating fog in a jar. The kids were super excited to try it at home! If you want to try, this is what we did:
Fill a jar with hot water.
Pour out all but one inch.
Put ice cubes on top.
We tried this a few different ways and couldn't quite get a perfect fog, but they were able to see the condensation pretty well!