Well, it's a gas cap. I would say it keeps gas from spilling out of the tank, but there's already a valve that keeps that from happening. Again, it's a GAS CAP. It screws on comfortably and is easy to screw off (hehe...I put 'screw off' in this). I would've given it four stars, but that fifth star is earned when it screws itself on and off and puts gas in the car for me.