Pencil

Something that everyone has a ton of in the beginning of the school year, then has none at the end. Some people hold contests to see who can keep a pencil the longest. If you give someone a pencil to "borrow" you won't get it back.

Guy who sits behind you: "Hey, can I borrow a pencil? I'll give it back to you later."
You: "I can't dude, I only have one more and it's because I picked it up on the desk."