Sublimation
Sublimation is when a solid turns directly into a gas without melting into a liquid first.
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.
Projects that explore sublimation
Sublimation means a solid changes directly into a gas, with no liquid stage in between. When you build a dry ice comet model, you can see sublimation happen as gas jets stream off the surface. Aiming a hair dryer at the comet simulates solar wind, and the jets shift direction as heat pushes gas away from the solid.
When a solid changes directly into a gas, the gas needs far more space than the solid did. Frozen carbon dioxide sublimates, meaning it changes directly from solid to gas. You can trap this gas inside a balloon tied around dry ice and watch it inflate as the solid vanishes.