The video below is of dolphins playing with the unique silver colored rings they have the ability to create underwater.

The explanation for how dolphins make these silver rings is that the phenomena are “air-core vortex rings”. Invisible, spinning vortices in the water are generated from the tip of a dolphins dorsal fin when it is moving rapidly and turning. When dolphins break the line, the ends are drawn together into a closed ring. The higher velocity fluid around the core of the vortex is at a lower pressure than the fluid circulating farther away.

Air is injected into the rings via bubbles released from the dolphin’s blowhole. The energy of the water vortex is enough to keep the bubbles from rising for a few brief moments of play time.

Besides being nice to look at (and a cool demonstration of fluid mechanics), this phenomenon also might throw some light on dolphin cognition, since the skill to create the rings is a bit subtle and tends to be taught from one dolphin to the next via careful observation and practice. 

Also intriguing is the report that they seem to be using sonar to locate the vortex in the water, since that would be a fairly amazing bit of audio analysis.