A fascinating phenomenon: when walking along a sunny corridor with windows on the south side, I saw my shadow walking with me upside down on the north wall. The sunlight was reflecting off the smooth polished floor. If I walked near the windows the shadow was inverted, near the wall the shadow was rightside up as expected. In between, two shadows appeared, one upright and the other not. Weird! I think it's just a matter of geometry, but I find it nontrivial to visualize or diagram.