Soil Porosity
Soil porosity is the measure of empty spaces between soil particles where water and air can hide.
Think of it this way
A jar packed with marbles has gaps between each marble where air sits. Pour water in, and it fills those gaps without moving the marbles. Sandy soil works the same way — loose grains leave big gaps, so water drains fast. Dense, packed soil has tiny gaps, so water moves slow and air gets squeezed out.
Explaining soil porosity by grade level
Soil has tiny gaps between its bits. Water slips into those gaps and hides there. Some soil has more gaps than other soil. Sandy soil has big gaps, so water moves through it fast.