Heat. It seems to be everywhere in one form or another. And wherever objects are at different temperatures heat transfer will occur.
In the modern world we spend a great deal of time trying to control heat transfer. Sometimes we try to slow it down, insulating our homes or stopping our freezers from defrosting themselves. Other times we try to speed it up, cooking food in fan assisted ovens or placing heat sinks in our computers to stop the processors overheating and destroying themselves. Sometimes though heat transfer is more than just a matter of convenience, ask any hill walker caught in bad weather without a 'space blanket' or a fire fighter entering a burning building to rescue a trapped child.
Probably one of the hardest tasks though falls to metallurgists and casting specialists who need to tightly control the rate at which a metal casting cools down. Different rates of cooling can produce very different castings with radically different properties, and if you find yourself flying in an aircraft, twelve kilometres above the surface of the Earth, your life is in the hands of the men and women who designed, developed and manufactured the components that make up the aluminium alloy tube that you are sitting in.
The following two questions should only be attempted after you have a good grasp of heat transfer. You might like to come back to them after you have read the rest of the heat transfer section.