Amitai Etzioni presents an essay by David P. Barash on whether or not it is reasonable to be reasonable. It's excellent, and you should read it.

One of the best parts is an elegant examination of something called the Wason test (after its inventor Peter Wason):

Imagine that you are confronted with four cards. Each has a letter of the alphabet on one side and a number on the other. You are also told this rule: If there is a vowel on one side, there must be an even number on the other. Your job is to determine which (if any) of the cards must be turned over in order to determine whether the rule is being followed. However, you must only turn over those cards that require turning over. Let's say that the four cards are as follows:

T 6 E 9

Which ones should you turn over?

Got that? OK, now think about this:

You are a bartender at a nightclub where the legal drinking age is 21. Your job is to make sure that this rule is followed: People younger than 21 must not be drinking alcohol. Toward that end, you can ask individuals their age, or check what they are drinking, but you are required not to be any more intrusive than is absolutely necessary. You are confronted with four different situations, as shown below. In which case (if any) should you ask a patron his or her age, or find out what beverage is being consumed?

patron #1 Drinking Water
patron #2 Over 21
patron #3 Drinking Beer
patron #4 Under 21

For the answers, see Barash's essay. I guess it's old news to undergrad psych students, but I thought it was just fascinating. You can dig into the significance of the Wason test, and take another version of it, here.