Answer: Heat transfer
Step-by-step explanation:
Hot material such as hot air or liquid is more dense that cool material so it will rise. As it does so the cool material will sink towards the source of heat that is making the hotter materials hot and get heated as well which will enable it to rise and the whole cycle repeats.
This type of heat transfer is called Convection and it is very important to processes occurring on earth from basic things such as how water is heated in a kettle to how rainclouds are formed in the atmosphere.