Moisture Absorption
Moisture absorption is what happens when a material soaks up water from the air around it.
Think of it this way
A dry sponge sits on a counter next to a bowl of warm water. The sponge pulls water from the damp air above the bowl into its tiny holes. As more water moves in, the sponge grows heavier and softer. This is how moisture absorption works — dry materials draw water from the air into their inner spaces.
Explaining moisture absorption by grade level
Wood pulls water right out of the air, even when nothing looks wet. When wood soaks up water, it gets a tiny bit bigger. When it dries out, it shrinks back down. You can see this happen with different types of wood.