The term weathering refers to any environmental process physical, chemical, or biological that changes rocks that are exposed at Earth's surface. Scientists define physical weathering as any kind of mechanical process that breaks down surface rocks. For example, the cycle of alternating freezing and thawing of water in rock crevasses (or cracks) breaks apart rocks, because frozen water (ice) takes up more space than liquid water. Tree and plant roots also force themselves into and between rocks to break them into smaller fragments or grains. Strong winds, especially in desert environments, may pick up sand grains, driving them into exposed rocks and eroding them and leaving marks called ventifacts (from the Latin words for wind and face). In chemical weathering, weakly acidic raindrops falling on rocks and sediments cause chemical reactions, such as dissolving halite and other salts. In biological weathering, lichens, moss, and microorganisms release acidic compounds that accelerate the breakdown of rocks and grains on which they are growing.