What type of example is this, "A thermometer works because the liquid in it contracts when heated."?

What exactly is convection? This is the process wherein heat transfer will occur. The less dense material will rise while those that are dense will start to sink. This is what happens with the thermometer, the mercury that is less dense will start to rise and in the process will show the temperature of the person.

There are also other examples of convection such as water boiling. It may start as room temperature but it would slowly boil. When ice melts, this is also another example of convection. A hot air balloon is another example because hot air is needed in order to make the hot air balloon rise up.