Students know natural processes, including freezing and thawing and the growth of roots, cause rocks to break down into smaller pieces.

The California Science Framework (page 64) distinguishes between chemical weathering and physical weathering. In both cases, the term weathering is used in its technical sense in geology meaning the breaking of rocks to smaller pieces. Rainwater and groundwater are generally somewhat acidic due to dissolved carbon dioxide from the air. The dissolved carbon dioxide forms carbonic acid, which is a weak acid. This acidity can chemically cause rocks to break apart or dissolve.

Plant roots that grow into cracks in rock can physically break rocks as they continue to grow. Freezing and thawing of water is another process that can physically break rocks. Students may not realize that water expands when it freezes. As recommended in the Framework, they can experience this effect by completely filling a soft plastic bottle with cold water, and then placing the tightly capped bottle in the freezer.