What if your clueless 16-year-old accidentally filled your car up with DIESEL fuel? Well, the good news is that probably won't happen . . .
The nozzles on diesel pumps are slightly bigger the ones on regular pumps. So unless you have a really old car, they won't fit in your gas tank. (Here's a video.) But if you did somehow get diesel fuel in your tank, it wouldn't be good.
Gas engines use spark plugs to ignite the fuel. Diesel engines do it with compression. That's one reason they need different types of fuel. And the two fuels really ARE different. It's not like Coke vs. Pepsi.
If you somehow ended up with diesel fuel in your car, it wouldn't run for very long. It might go for a while if there was still some gas mixed in there. But pretty soon it would start making weird noises. And eventually it wouldn't work at all.
Best case scenario, you'd have to flush the whole system, and replace some parts. Depending on the car, that might cost between $500 and $1,500. Or you might have to replace the whole engine. Obviously that would cost a lot more.
So if you ever do end up with diesel fuel in your car, don't drive it. Turn the engine off, and have it towed to a mechanic. Otherwise you'll just make the damage worse.