Two guys are driving through the desert

Two guys are driving through the desert when their car breaks down thirty miles from the nearest town. The driver gets a big backpack from the trunk, the passenger gets a wrench and takes the passenger door off and they start walking.

After a couple of miles the guy with the door asks why are you carrying that bag? It looks heavy. He said it has snacks and water in case we get hungry or thirsty.

After a couple more miles, bag man asks why are you carrying that heavy door through the desert?

He said if we get too hot I can roll down the window.