Newspapers are spread over tabletops and floor tiling to catch any stray dirt, sand, and gravel that might spill as students investigate "nature's underground washing machine"-that is, how rainwater that seeps into the ground is made drinkable again. All around the room there are exclamations of excitement. "Cool! Hey, pour some more dirt in here." "Shelly, will you pass the gravel?" "Wow! Look at how different the water looks now!" Over the din the teacher says, "Five more minutes until clean-up."