Erosion
Erosion is when wind, water, or ice wears away rock and soil and moves it to a new place.
Think of it this way
When you rinse a sandy bowl, water pushes the sand grains off the surface. The water carries those grains down the drain, away from where they started. This is like erosion — wind or water lifts loose bits of rock and soil and moves them elsewhere. Over time, even big rock layers wear down this way.
Explaining erosion by grade level
Think about your pet rock sitting outside in the rain. Over time, rain washes tiny bits of rock away. Wind blows small grains off too. Bit by bit, the rock gets smaller and the pieces end up somewhere new.