In warm water, manatees may be seen by the parts of their bodies that briefly appear at the surface. Their snouts emerge when they come up to breathe air, with nose flaps that open and close to keep water out of their lungs, and when resting they may rise every 7 to 20 minutes to breathe. On clear days, their backs can look like oval-shaped rocks with barnacles near the surface. Their gray, paddle-shaped flippers may wave and splash as they steer and hold vegetation while eating, and the bumps near the ends are fingernails left from when their ancestors lived on land millions of years ago. Their large, fan-shaped tails propel them through the water, and when they submerge they leave half-moon swirls on the surface.