I've not seen this trick before, but Zach Weiner at Saturday Morning Breakfast Cereal is correct. Not only is he correct, but his math is correct, and his philosophical punch line is funny and correct. And I'm not saying that just because I'll see him at Comic Con soon and I'm trying to get him to buy the first round. I swear, sometimes when I don't get a math trick, that guy in the panel really is me. Also. Don't forget to hold your mouse over the red button at the bottom for extra bonus Zach-ish goodness*.
^* He really does look like that.