Re:I remember this idea from years ago (Score 5, Informative)

The principle that was actually at work in the window-paddle was not, in fact, the conversion of the photons' momenta into kinetic energy of the paddle, which is the principle behind solar sails, but the black side of the paddle would absorb the photon, increase in temperature and warm the air just above the black surface. The air would then expand, causing the paddle to rotate.