Filtering

Sandy, salty water is poured through a filter. Does the paper catch the sand? Does the paper catch the salt?

The paper catches the sand, but the salt goes through.

The rule

Filtering — pouring a mixture through filter paper. The paper has holes too small to see.

Solid bits that are bigger than the holes get caught.

A dissolved solid is spread through the water in tiny pieces. The pieces are smaller than the holes, so they pass through. The paper cannot catch it.