Over the weekend, I asked about the history of military formations, and invited commenters to help with my schooling. The thread is one of our best ever, and worth reading. I downloaded Total War yesterday, but haven't been able to play much because I've been working. I did do the tutorial and found the incorporations of history fascinating. You're playing the game but you're also learning how archers and calvary actually worked in the past. (Assuming the game's accuracy.)

Anyway, this response said a lot:

This thread makes me envy TNC's ability to ask a question on his blog and get several intelligent answers in a matter of minutes.

Yes. Envy me. And now to make my terrible power manifest: Each week, I'm going to ask about something that I know absolutely nothing about, but am insanely curious. I'll invite you all to give an answer. The next day we'll pick a winner. (I haven't decided whether to do it on "likes" or to be an autocrat. I lean toward dictatorship. Any thoughts?) 

The prize will be one guest post, the following week, covering anything the winner wants. We'll start tomorrow. The topic will be financial derivatives. The person who can best make me understand them, in clear English, wins. Do not put your answers below. We'll start tomorrow.

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