Sharks do not have “swim bladders”-the gas-filled balloonlike organ that enable most fish to stay afloat and upright in water. This means that, since a shark’s body is heavier than water, it will indeed sink when not swimming. Some sharks, including great whites and hammerheads, actually have to keep swimming to breathe. They need to constantly move forward in order to pass oxygen-bearing water over their gills-a process called obligate ramjet ventilation. But there are many species of shark that do not have to swim to breathe. Other common misconceptions about sharks are that they don’t have eyelids and never sleep. Fish have no eyelids, but sharks actually have elaborate eyelids and some even have an additional, protective eyelid called a nictitating membrane. There is some debate among marine biologists as to whether sharks actually sleep or not, but most think that sharks do slow down their brain functions from time to time and go on a sort of “autopilot.” In fact, sharks may swim in their “sleep,” although it’s not the sort of snoring, drooling, butt-scratching snooze we’re accustomed to.