There are many kinds of soil. Each kind of soil has big and small rocks in it and some humus. If rocks in a soil are as big as your fingers, we call those rocks "gravel". Smaller rocks are called sand. Very small rocks are called silt. Very, very small rocks are called clay. You can see sand with your eye. Sand feels rough between your toes. Silt rock is very small and you must use a lens to see silt. Silt rock is smooth between your toes. Clay rock is too small to see with a lens. To see the smallest things you must use a big microscope. You must use a big microscope to see clay too. Clay rock feels slippery between your toes. Most soils contain all kinds of small rocks. The three best rocks for making soil are sand, silt, and clay.