While some new-age crystal-fanciers no doubt assumed that our cetacean friends were already good pals, this video from the American Museum of Natural History shows rare photos of a specific type of 'playful' behaviour - dolphins getting lifted up into the air, riding a humpback whale's head.

From the AMNH:

In two different locations in Hawaii, scientists watched as dolphins "rode" the heads of whales: the whales lifted the dolphins up and out of the water, and then the dolphins slid back down. The two species seemed to cooperate in the activity, and neither displayed signs of aggression or distress. Whales and dolphins in Hawaiian waters often interact, but playful social activity such as this is extremely rare between species.

This behaviour was documented in the journal Aquatic Mammals by Deakos et al. (2010). Animal Wise has a great review of the paper here, as well as a video of a dolphin hitching a ride:

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