transfer of heat by a movement of liquid and gas due to different density levels which is caused by the difference in temperature. An example of this would be when a kettle is being heated up by a stove. The bottom of the kettle is heated first then the heat rises to the top and the cooler water flows down to be heated.
The water molecules at the bottom of the kettle gain heat energy from the flame and vibrate faster and move further apart. Their density decreases and the hotter particle rise to the top of the kettle. The colder less dense molecules move into replace the hotter particles. This continues until all the water is the same temperature.