Trees and plants commonly weather earth materials in a process called root-wedging. This occurs when a root grows into an already existing crack in a rock. As the root grows, it creates pressure in the crack; eventually, the root grows so large that it widens the crack and sometimes breaks the rock apart. Additionally, small plants such as grasses often grown in cracks, eventually breaking rock apart; this process is commonly seen in disused sidewalks.
Animals commonly break rocks and soil into smaller pieces. Burrowing animals such as moles and worms often "abrade" rocks and soil because they move small pieces of rock against each other; this process is known as bioturbation. In addition, large animals often break down soil simply by walking. Human beings are engaged in organic mechanical weathering when they engage in large-scale construction projects; even kids throwing rocks down by a stream can result in mechanical weathering, as the thrown rocks often chip and break apart.
Although lichens are primarily engaged in chemical weathering, they also weather rocks by mechanical processes. Lichens contain root-like organs called rhizines. Lichens use rhizines to penetrate cracks in rocks and anchor themselves. This can break rocks apart in a process not unlike root-wedging; it simply occurs on a smaller scale. This is one type of organic mechanical weathering that often affects large rocks not susceptible to other forms of organic mechanical weathering.
Lichens also help to mechanically weather rocks by inadvertently aiding the process of physical weathering known as "slaking." Slaking occurs when rocks become alternately wet and dry. As the number of water molecules between the grains in a rock increase, the grains are pushed apart with increasing force. Because lichens are capable of absorbing large quantities of water, they often exacerbate the effects of slaking on the rock surfaces on which they grow.