system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: New driver here, can anyone explain the importance of changing your car's engine oil and why the engine should have it's coolant flushed?
gpt
Your engine has metal parts that rub against each other (metal pistons go up and down inside metal cylinders.) This creates friction, which creates heat. If these parts heat up too much, they start to deform or change shape (hot metal gets soft and expands).

Engine oil both lubricates (reduces Friction) and reduces heat (many cars circulate this oil to an oil cooler). This oil prevents these parts from deforming.

Over time, the high heat starts to degrade the oil, limiting its ability to lubricate. So it needs to be swapped out for new oil. Some oil (synthetic) can be swapped at longer intervals than other oil (natural). Also, microscopic bits of metal from all the moving parts get into the oil and cause extra wear (imagine rubbing your face with water vs. water with some sand in it. The oil filter catches this and thus needs to be changed occasionally too.

The engine coolant keeps the engine cool and is important, but doesn't need to be changed very often. Mainly because it can get full of rust and mineral scale deposits which can eventually clog your cooling system.