We hiked to a local hot spring last week and my 8yo was kind of floored by how warm the water was (he happily stripped down to skivvies and swam!). The best description I could come up with at the moment was lava under the ground was warming up the water. I don't know if he believed me, so we're off to Google to find out what exactly makes a hot spring hot. .
Anyone know of a great book that explains geological things like this that's at an elementary reading level? One with great pictures is a plus.