Acid rain corrodes, or wears away, rocks, metal, and other materials. Plants are also known to produce weak acids that dissolve minerals. There are several different agents, or causes of mechanical weathering. Temperature, frost action, organic activity, and abrasion are examples. Temperature variations over a period of time allow the rock to expand and contract repeatedly causing curved-shaped pieces to break off.
Frost action occurs as water seeps into tiny cracks in the rock and freezes at night.
As the ice expands it breaks rock fragments free. Organic activity occurs as plant roots slowly pry apart the rock as the plant grows larger. Abrasion caused by blowing winds weathers rocks by rounding sharp and protruding edges to smooth surfaces. A discussion of the breakdown of rock would be a good way to begin this experiment. The formation of the earth from it's molten rock beginnings to it's current form indicates that changes have taken place.