Car door.

Three men are in the middle of a desert when their car breaks down.

For their hike to town, they each decide to take one thing with them.

One man takes a jug of water.

The second man takes a sandwich.

The last man takes one of the car doors.

The first man says to the last man: Im bringing the water because if I get thirsty, I can take a drink. And it makes sense to bring a sandwich in case we get hungry, but why bring a car door?

The last man replies, If I get hot, I can just roll down the window.