Weathering
Weathering is the slow breaking down of rocks by water, wind, ice, and living things.
Think of it this way
A bar of soap sitting on the edge of the tub breaks down slowly over time. Water flows over it, bits flake off, and the sharp edges round out. Wind, rain, and ice do the same thing to rocks over many years. The hard surface wears away piece by piece until the shape changes.
Explaining weathering by grade level
Rain falls on a big rock year after year. Tiny bits chip off and wash away. Ice forms in small cracks and pushes them apart. Over time, the hard rock turns into soft, loose soil.