Engines are made from metal, because it's one of the few materials which can withstand the forces of the explosions, withstand the high temperatures and which can be made into the right shapes. The cylinder is made of metal. The piston and piston rings are made of metal. What stops this from scraping against each other in a nasty way?
The answer is oil. If you've got two metal surfaces rubbing against each other you need at least a thin layer of oil between them. If you run out of oil, or the oil film breaks at some place in your engine, the metal will rub together and get hot very quickly, potentially welding themselves together.
Why is oil good for this purpose? Most obvious, it's slidy. If you have two metal surfaces sliding against each other, then putting a layer of oil between them will decrease the friction.
What is oil made from? What makes it slidy? Long molecules? Why doesn't water work?
It has to be stable at temperature - can't decompose quickly.
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