The "tickle machine" at work on Alan.

Buttoning a collar or putting on a necklace, you probably brush your fingers against your neck every day without taking notice. But the sensation of a friend's fingers will most likely make you squirm and laugh. What's the difference? Why can't you tickle yourself?

The popular explanation holds that tickling is a social interaction, so by definition, you can't do it alone. In "A Ticklist Question," Alan Alda visits with psychologist Christine Harris, who uses her tickling "machine" to turn that conventional wisdom on it's head. A blindfolded Alan has the sole of his foot tickled by Harris' tickle machine, rating the sensation a 7 out of 7 on the Tickler scale. The machine (actually a well-hidden grad student) shows that people can be tickled even by what they think is an inanimate object. Therefore, Harris reasons, tickling has little to do with social interaction.

The tickled often laugh even while trying to escape

She is also investigating another popular notion - that tickling makes us laugh because we think it's funny. As all stand-up comics know, laughter begets laughter; that is, it takes a few jokes to get an audience really going. Harris argues that if tickling and humor are related, hearing funny jokes before being tickled should enhance ticklishness. In her experiment, Harris found that people "warmed-up" with a funny video were no more ticklish that those who had watched a nature documentary. What's more likely, she concludes, is that tickling creates nervous rather than humorous laughter. Hence the reason why even people (like Harris) who hate to be tickled will laugh when someone strokes the bottom of their feet.