Good illusion

What's the difference between the shapes of these two table tops?

Obviously (because this is an illusion) the two are exactly the same. Go on, measure! Apparently it's the legs that are the primary influence.

I grabbed this from Beyond Belief (talk #7). The whole thing I highly recommend as a way to fritter away 17 hours of your life. Intelligent debate (mostly). No whooping.

UPDATE: the same talk links to a fascinating online questionnaire highlighting certain innate prejudices we "all" have. For example, to categorize something as, say (fat or black) vs. (thin or white) is a task that can be done much faster than (fat or white) vs. (thin or black). Truly enlightening and I urge everyone to have a go.