Build a "house" on a hill of sand: Use your sandbox or a nearby park or just the dirt in your backyard (you could buy play sand and put it in a box - if you have time and money to go out and buy some). Let the kids form the sand or soil into a soft mound. Let them build their house usingblocks, sticks, cardboard, paper, or something else that's ok to get wet. The goal is to make the house fall when the sand washes away, so something like Legos may not work. Pour enough water over the house and sand that the mound of sand/soil washes away and the house tips or falls. Now try it on a solid surface, preferably a large rock if available, but concrete, tile, or something similar will also work.