Sublimation
Sublimation is when a solid turns directly into a gas without melting into a liquid first.
Think of it this way
A piece of dry ice sits on a plate in the kitchen. White fog drifts off it, but there is no puddle underneath. The solid chunk shrinks and shrinks without ever turning to liquid first. It goes straight from solid to gas, skipping the melting step entirely.
Explaining sublimation by grade level
Think about a piece of dry ice sitting on a table. You can see white fog floating off it, but there is no puddle underneath. The solid piece gets smaller and smaller, but it never melts. It goes straight into the air. The solid skips being a liquid and becomes a gas all on its own.