Weathering, the process of breaking down rocks, has helped shape the earth since the beginning of time. And it happens in two different ways. Mechanical weathering (also called physical weathering) occurs when plant life, temperature change, water, and wind break down rock into tiny pieces. Chemical weathering, on the other hand, happens when water mixes with another substance to change a rock's composition causing it to weaken and break. Both chemical and mechanical weathering break down rock into small pieces but in neither case does the rock move. When rock moves, a different process takes place. It's called erosion.