I once visited one of those tunnel aquariums and caught the rays in the act. They were hunkering down on one side of the tunnel and then launching themselves across it as humans approached. Because of the way the tunnel was designed, it looked like they were coming *right at your head*.

It was impossible not to duck. There was no possible motivation for this behavior other than play. "Let's make the humans duck. It's funny." We don't think of fish being smart and social enough to engage in play, let alone *play pranks on us*, but there it was.