center of the bubble. If the fluid jet has enough energy, it will puncture the top of the bubble and create a bubble ring. Because of the motion of the fluid moving through the center of the bubble, the bubble begins to rotate. This rotation moves the fluid around the bubble creating a toroidal vortex. If the surface tension of the fluid interface or the viscosity of the liquid is too high, then the liquid jet will be more broad and will not penetrate the top of the bubble. This results in a spherical cap bubble. Air bubbles with a diameter greater than about two centimeters become toroidal in shape due to the pressure differences. Cavitation bubbles Cavitation bubbles, when near a solid surface, can also become a torus. The area away from the surface has an increased static pressure causing a high pressure jet to develop. This jet is directed towards the solid surface and breaks through the bubble to form a torus shaped bubble for a short period of time. This generates multiple shock waves that can damage the surface. Cetaceans Cetaceans, such as beluga whales, dolphins and humpback whales, blow bubble rings. Dolphins sometimes engage in complex play behaviours, creating bubble rings on purpose, seemingly for amusement. 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 creating a toroidal vortex with their flukes and injecting a bubble into the helical vortex currents thus formed. The dolphin will often then examine its creation visually and with sonar. They will sometimes play with the bubbles, distorting the bubble rings, breaking smaller bubble rings off of the original or splitting the original ring into two separate rings using their beak. 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. Dolphins also have the ability to form bubble rings with their flukes by using the reservoir of air at the surface. Humpback whales use another type of bubble ring when they forage for fish. They surround a school of forage fish with a circular bubble net and herd them into a bait ball. Human divers Some scuba divers and freedivers can create bubble rings by blowing air out of their mouth