In "Ideas for Discussion," we mentioned that potholes are formed when freezing water expands and cracks the asphalt. In this experiment we show how water can break glass by expansion during freezing. Children can then infer that the same thing can happen with water and rocks in nature.
1 quart jar and lid (spaghetti sauce or mayonnaise)
In front of the class, fill the jar with water and seal it with the lid. Wrap the towel around the jar and freeze it overnight.
Unwrap the jar in front of the class and ask for their observations. What happened to the jar? (The water expanded as it froze, forcing the glass to crack.)
Explain to the class that in nature the same thing can happen when rainwater gets inside the holes and cracks rocks. This small but constant process slowly erodes the rocks and changes the way they look.