2nd law of thermodynamics says that, heat will always flow spontaneously from hot to cold, and never the other way around. A refrigerator causes heat to flow from cold to hot by inputting work, which cools the space inside the refrigerator. The coolant/ refrigerant liquid goes around in cycles of being evaporated into gas (absorbing heat and cooling the air) then being compressed into liquid again (giving out heat to the surrounding, probably why the back of a fridge is usually warm) And engines are machine that converts energy from electricity or some other kind of source to a mechanical energy/ work. And in a way A refrigerator is a heat engine run backwards.