Here's why:

* Physical Weathering: Water can freeze and expand in cracks, causing the rock to break apart (frost wedging). Water can also dissolve minerals in the rock, weakening its structure.

* Chemical Weathering: Water acts as a solvent, dissolving minerals and breaking down rock. It also acts as a catalyst for other chemical reactions, like oxidation.

* Biological Weathering: Water is essential for the growth of plants and organisms that can contribute to weathering. Roots can grow into cracks and break apart rocks, while organisms can produce acids that dissolve rock.

Other factors that contribute to weathering:

* Temperature: Fluctuations in temperature can cause rocks to expand and contract, leading to cracks and weakening.

* Acids: Acid rain, caused by pollutants, can accelerate chemical weathering.

* Wind: Wind can carry abrasive particles that can wear down rock.

However, water plays a crucial role in all of these processes, making it the most significant factor in rock weathering.