Demonstrate how parent material can be broken into smaller pieces by rubbing two pieces of sandstone together over a white piece of paper. Particles of sand will fall off. Explain to students that you are using sandstone because it is easier to experiment with, other rocks need more force and time. Nature provides for these over time in the environment, but your school day will end in a few hours, not a few centuries. Place the sandstone into some water, rub the stone with your finger.
Particles come off.
You could demonstrate how water running over the stone will also wash away mineral particles, but using your finger conserves water. Water erosion on rocks takes gallons of water and many years. Your school day will be over soon. Show a river rock, note the smoothness. Heat a small piece of sandstone or limestone on a hot plate.