Every time I sort clean clothes I get upset with the math community. I realize that sounds odd at first, but bear with me.
More precisely, it is not the mathematical community per se but the nerdy logic puzzle people who upset me. Unfortunately, these are not mutually exclusive categories. The mathematical logicians haven't done anything (or not enough) to quash the insidious warping of intellectual capacity.
There is a problem posed, over and over, something like this: "You have 10 pairs of black socks and 10 pairs of white socks, except that they are just now tumbling out of the dryer and so they aren't actually pairs at all but just 20 socks of which half are white and half are black, so my saying that you had pairs of socks was really not meaningful at all. For some mysterious reason you can't actually see any of them right now as they come out of the dryer, and it is late so you need to grab some socks quickly. Of course, later on you will be able to see the socks you chose." (I've never quite understood the nature of this temporary blindness, but in the math puzzle world you are expected to suspend credulity. Anyway, that's not what upsets me.) Now for the question: "How many socks should you pull out of the dryer in order to be certain that you have at least one matching pair?"
The answer is 11, one more than half the socks.
Oh, I can hear them screaming already, and I haven't even saved the text yet. Patience, little minds, patience.
They will tell you, falsely, that you only need to pull out 3. They truly believe this, which is why you so often see math nerds wandering around with a low-cut sock on the left foot below a hairy, pale left leg and a knee sock covering their right calf. If you question them, they will stare at you blankly and say, "My socks match. They're both white." Or black.
In actual reality 2 white (or black) socks do not necessarily make a matching pair, even if they are both your own socks. The socks may be of different lengths, different styles, different yarns, different knits, different amounts of wear. They may even be different shades of black (or white), especially if they are of different ages and have been washed a different number of times.
If you want a matching pair, 3 socks isn't always going to do the job. Drawing 3 socks -- given that there are only 2 colors -- will always insure that you have 2 of the same color.
So what?
Examples of this puzzle appear ubiquitously in the logic puzzle literature.
I offer a few currently available examples from the internet:
• http://en.wikibooks.org/wiki/Puzzles/Logic_puzzles/Pair_of_Socks/Solution
• http://www.gymnasiumforbrain.com/039drawingapair.htm
• http://www.easycalculation.com/puzzles/logical/logical1.php
• http://www.jimloy.com/puzz/socks.htm