One of the ways in which rocks are weathered (broken down into smaller pieces) is by falling water.
Place a bar of soap on a sponge. Set the sponge in a sink under a faucet.
Use the faucet as usual for a day, letting the water hit the soap and observe the soap at the end of the day. Or you could let the faucet drip on the soap for the duration of the class period.
The falling water knocks particles of soap free. Similarly, rocks at the bottom of waterfalls (or in other locations where water falls over them) are weathered by the falling water over time. Of course, this process takes a very long time since rocks are lots harder than soap!
