A stranger meets you and shows you a wad of cash he has just "found." He says he wants to split it with you, but says that, in order to show your "good faith," you have to put up some of your own money. When you agree to give him your money, the stranger finds a reason to leave for a while. Do you see him again?
Probably not - you have just been conned.

The main theme of this passage is __________.