Convection
Convection is the way heat moves through a liquid or gas by making warmer parts rise and cooler parts sink.
Think of it this way
A pot of soup on the stove shows convection in action. The broth near the burner gets hot and rises to the top. Cooler broth along the sides sinks down to take its place. This loop keeps going, carrying heat through the whole pot without anyone stirring.
Explaining convection by grade level
When you heat water in a pot, the water at the bottom gets warm first. Warm water is lighter, so it floats up. Cooler water from the top sinks down to take its place. Then that water gets warm and rises too. The water keeps going around and around, carrying heat with it.