air-core vortex rings or "bubble rings". There are two main methods of bubble ring production: rapid puffing of a burst of air into the water and allowing it to rise to the surface, forming a ring; or swimming repeatedly in a circle and then stopping to inject air into the helical vortex currents thus formed. The dolphin will often then examine its creation visually and with sonar. They also appear to enjoy biting the vortex-rings they have created, so that they burst into many separate normal bubbles and then rise quickly to the surface. Certain whales are also known to produce bubble rings or bubble nets for the purpose of foraging. Many dolphin species also play by riding in waves, whether natural waves near the shoreline in a method akin to human "body-surfing", or within the waves induced by the bow of a moving boat in a behavior known as bow riding. Cross-species cooperation There have been instances in captivity of various species of dolphin and porpoise helping and interacting across species, including helping beached whales. Also they have been known to live alongside resident (fish eating) orca whales for limited amounts of time. Dolphins have also been known to aid human swimmers in need, and in at least one instance a distressed dolphin approached human divers seeking assistance. Creative behavior Aside from having exhibited the ability to learn complex tricks, dolphins have also demonstrated the ability to produce creative responses. This was studied by Karen Pryor during the mid-1960s at Sea Life Park in Hawaii, and was published as The Creative Porpoise: Training for Novel Behavior in 1969. The two test subjects were two rough-toothed dolphins (Steno bredanensis), named Malia (a regular show performer at Sea Life Park) and Hou (a research subject at adjacent Oceanic Institute). The experiment tested when and whether the dolphins would identify that they were being rewarded (with fish) for originality in behavior and was very successful. However, since only two dolphins were involved in the experiment, the study is difficult to generalize. Starting with the dolphin named Malia, the method of the experiment was to choose a particular behavior exhibited by her each day and reward each display of that behavior throughout the day's session. At the start of each new day Malia would present the prior day's behavior, but only when a new behavior was exhibited was a reward given. All behaviors