Photon Pressure?
Way back in 1873, some dude named Sir William Crookes noticed some weirdness in a scale he built. It appeared as though some samples weighed more or less depending on if sunlight was shining on the scale. Weird! He postulated that it was the pressure of the light being exerted on the scale that modified his results.
Of course, he was totally wrong, but it was a cool idea.
See, the bulb in which the blades spin is a partial vacuum. Partial being the tricky part - another clever scientist by the name of Lebedev noticed that the effect disappeared in a hard vacuum. So, air has something to do with it. Basically, the principal is the air that hangs out by the cool side of the blade flows slowly to the warm side of the blade. That process is called thermal transpiration. Science is cool.
The ThinkGeek Radiometer looks awesome sitting on your desk or windowsill. It works great in direct sunlight, but moves pretty well even hit with a flashlight! Of course, in ThinkGeek's own highly scientific testing, a laser, while intense, was too focused to significantly move the vanes very quickly at all. Your mileage may vary!
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