Sharks have a skeleton made of light-weight cartilage, rather than calcified bone. When they swim, by beating their tail fin back and forth, the flexible skeleton allows their body to bend. Because they lack internal sacks (gas bladders) for buoyancy, they must swim constantly to stay afloat. Sharks use their stiff, pectoral fins to provide lift, like airplane wings. It turns out that sharks also have a fluid-based (hydrostatic) skeleton. Muscles attach directly to their skin, and the skin acts like the outside of a balloon, withstanding high pressures as muscles contract. The pressure in a swimming shark can rise as high as twice that in a full car tire. Crisscrossing layers of the protein collagen make shark skin tough and rigid to withstand the pressure. Sharks have been equated with swimming balloons.