My Siamese Flying Fox does something similar. I have an L-shaped artificial hollow log, lying on it's back - and Noel (so-called coz he/she's a coward...) swims out the long side, up over the top, down through the short side over and over again. Eventually he/she will come to rest head-first down the short side (doing a hand-stand) and all I can see is a tail peeking out the top!!!