(Although all of these go back to philosophy in one way or another, it's more for fun than an actual discussion, so here it is in senseless. Edit: On second thought, this should probably be in the Lounge. I hate second thoughts.)

Just throwing a few paradoxes in here for you all to think about. I always find unravelling the structure and exactly where the paradox is created to be great fun, so puzzle away.

Protagoras is educating Euathlos to be a lawyer. They've made an agreement on when Protagoras will be paid, and that's as soon as Euathlos wins his first case. But after his study, Euathlos doesn't become a lawyer. He doesn't take on any cases, and refuses to pay Protagoras because he has not won his first case yet. Protagoras wants his money, and decides to take his claim to court.
He argues: "If I win this case, Euathlos will have to pay me the money for his study, as per my claim. Yet, if I lose, Euathlos wins his first case case and will also have to pay me. Either way, I get my money."
Euathlos argues in the exact opposite fashion: "If I win this case, it Protagoras' claim is refused and I will not have to pay him. Yet, if I lose, I will not have won my first case and I also do not have to pay."
Who is in the right here?

A teacher announces to his class that the test for this term will be a surprise test.
"You will have to study up and have the knowledge at hand always, because I guarantuee you when you get this test, it will be a surprise!"
Bob and Silvia discuss the matter after class. Bob is particularly bummed out, this subject is not his favorite. But Silvia cheers him up:
"Don't worry, I don't think there will be any test at all. I think the teacher is just joking."
"Really? Why?"
"Well, consider this. He couldn't give the test on the last day, because everyone would know the test has to be given by then, so it will not be any surprise."
"Well, great. How does that help us?"
"Well, if we've eliminated the last day, we now turn to the second-last day. As we know he can't give the test on the last day, when the second-last day comes, it will have to be given by then. But that won't be a surprise anymore!"
Now Bob gets it too: "Oh, and that also rules out the third-last, fourth-last, and every other day! He can't give the test!"
Fully confident, Bob and Silvia refuse to study, untill three weeks later the test is suddenly given. Bob is shocked:
"But teacher, you can't give the test! It has to be a surprise!"
"Yes Bob, you weren't expecting any test, and still I give it. Isn't that quite a surprise?"
Were Bob and Silvia wrong, or was the teacher?

"This santence contains two errors."
What is going on with the above sentence?