To kill water bugs?
When I was 17 years old, I worked as a camp counselor for a camp that took inner city kids to a camp in the woods. My first group (with a co-counselor) was seven 5-7 year old boys. We decided to take them on a "real-live" camping trip for a weekend (trying to work out some of that energy!), out it the woods. I decided to make a bang-up fire pit and used some flat rocks, like sand stone, taken from a near-by dry creek bed to line it. It was lovely, and I figured it would maintain heat late into the night.
So, we hunted firewood and hot-dog sticks, built a big old bonfire and sat back - until the first explosion. Serious explosion with flying rock- first of about 12 (seemed like 100). Screaming kids, all of us running - luckily, no injuries.
Seems that sandstone carries water in it, enough water that when heated can break rock. Whoo boy, good times!!! My first lesson in heating sealed - or at least semi-sealed - objects.
