Who's magically filling your tank when you're not looking? Combustible engines aren't some magical force, they're a well known human advancement. I feel like even the layman knows in principal that pistons compress petrol into a gas, then ignite that gas with a spark plug causing a tiny explosion forcing the piston back down, generating movement. Obviously as things get burned up they change chemical composition and can no longer be used for the original purpose, it's a pretty fundamental understanding. When you put a log into a fire, you can't use that log infinitely, you have to keep adding fuel to the fire.