Headline! Radio buttons originally controlled radios
If you've read About Face, you know about how to use radio buttons. But if you haven't read Tog on Interface, you don't know how they were invented, or why they're called that. You really don't need to, but it's damn interesting, and while the book is outdated, Tog is funny as hell. Actually, it may be useful too; a friend just wrote to ask, 'Is it radial or radio?' The reason it's radio is because it mimics the behavior of those old analog car radios. They had a row of buttons to select a station, and pushing one in moved the analog needle to the preset frequency. Of course you can only select one station at a time, hence they're a good metaphor for when you need a mutually-exclusive UI select widget. Also, you can plainly see which choice is selected and all the choices at a glance.
Hmmmm, I feel old.