When you're in college (and often in high school), classes are often graded on a curve. This means, generally, that scores will be adjusted to account for the difficulty of the assignment; professors can't always write perfect tests.
And about once a year, at the end of a quarter/semester, you'll notice that, in one of your classes, everyone's talking about the final. Someone's had a light bulb moment, and now there's some brilliant idea going around the class, and eventually you get someone to tell you what it is.

We Could Skip The Final!

The reasoning invariably goes like this (in an excited voice):

1. He's grading on a curve, right? So no matter how low the average score is, the average grade will be the same, right?
2. So, like, if everyone got a 0 but one guy got a 5% that guy would get an A!
3. And if everyone got a 0, we'd all get A's!1
4. So we're gonna get everyone to skip the final!

And the professor shakes his head sadly and says it wouldn't really work like that, but can never give a satisfactory explanation.
This year, though, this happened in my computer science lower-division logic class. And the professor, being a rather smug person to begin with, said, "Sure! If none of you show up, I'll give you all A's."

There was a hushed silence.
"Of course," he added, "you'll never convince everyone to not come."
Naturally, most of the class started arguing: "Well, of course they'll stay home! They'll get an A!" Or, "Fine, we'll hire some bouncers to stand outside the door, then!"

But if you think about it, he's right. It's a classic example of the Prisoner's Dilemma. The greatest good for the community will be obtained if everyone stays home, but nobody can trust that every single other student will stay home. And if even one person comes in to class, the whole thing is ruined - everyone gets an F extept that one serlfish jerk, who gets an A for answering even one question right. So everyone says to themselves, "Well, I'm not going to be a sucker. I'll go take the test." They have to act in their own best interest, because they expect everyone else to do so too.
I'd love to stick it to that smug bastard professor, but of course he's right. We can't trust everyone to not come. I expect I'll see everyone at the final next week.

1 Of course, this is an interesting idea. But really that'd make the maximum possible score a 0, and everyone's personal score would be a 0. 0/0 isn't really an A, and there's no checkbox in the grading program for "indeterminate form". If the whole class got a 0/0, we'd risk destroying the universe with a divide by 0 error. It's a small risk, to be sure. But just in case, I'll go to the final.