Evaporation
Evaporation is when a liquid turns into a gas and moves into the air, even without boiling.
Think of it this way
A pot of water sits on the stove with the lid off. The water molecules near the top have enough energy to break free and float up into the air as invisible vapor. You do not see them leave, but the water level slowly drops. The same thing happens to a puddle on the sidewalk on a sunny day — no boiling needed.
Explaining evaporation by grade level
When you mix salt, bluing, and ammonia in a dish, the wet liquid slowly disappears into the air. That is evaporation. As the liquid goes away, the salt and bluing have nowhere to go, so they stack up into tiny, beautiful crystals. The water left, but the solid parts stayed behind.