If fluid near the bottom is heated, it expands. This reduces its density. This causes the hot fluid to floats up.
Colder fluid at the top is denser so it sinks.
The rising (lower density) fluid and the sinking (higher-density) fluid set up a convection current.
Colder fluid at the top is denser so it sinks.
The rising (lower density) fluid and the sinking (higher-density) fluid set up a convection current.