Introduction: Solid as a rock, the saying goes. If something is solid as a rock, it is unbreakable or immovable. But, rocks do break. It's part of the Earth's natural process of recycling its materials. The process of breaking down rock is weathering. 

There are two main types of weathering: mechanical and chemical. Mechanical weathering is the physical breakdown of rocks. The rocks get smaller, but they do not change in composition. Chemical weathering changes the mineral composition of the rock. Organisms, like plants and animals, can also cause rocks to weather both mechanically and chemically. Both types of weathering often work together, increasing the rate that rocks break apart.