Steam and Water Vapor
Steam and Water Vapor is water that has turned into a gas from being heated.
Think of it this way
A pot of water sits on a hot stove. The water near the bottom gets heated and its tiny bits move faster and faster. They rise up, leave the liquid, and spread out above the pot as loose bits floating in the air. Those floating bits are water vapor — the same water, just in gas form.
Explaining steam and water vapor by grade level
Heat water in a pot on the stove. Tiny bubbles form and rise to the top. A white cloud floats up from the pot. The water got so hot that it turned into a gas.