Water Filtration
Water filtration is passing water through layers like sand and charcoal to trap dirt and make it clean.
Think of it this way
You pour muddy water into a colander stacked with layers of coarse sand, then fine sand, then charcoal at the bottom. Each layer catches different sizes of dirt — big chunks stop at the top, tiny particles get trapped lower down. Clean water drips through into the bowl below.
Explaining water filtration by grade level
Dirty water can pass through sand and rocks. The sand traps bits of mud and dirt. Clean water drips out the bottom. Each layer catches things that are too small to see.