A seaplane is an aircraft designed to take off and land (correctly, though less commonly termed, "alight") upon water. There are two types of seaplane: the floatplane and the flying boat. A floatplane has slender pontoons mounted under the fuselage. Two floats are common, but many float planes of World War II had a single float under the main fuselage and two small floats on the wings. Only the "floats" of a floatplane normally come into contact with water. The fuselage remains above water. Some small land aircraft can be modified to become float planes. In a flying boat, the main source of buoyancy is the fuselage, which acts much like a ship's hull in the water. Most flying boats have small floats mounted on their wings to keep them stable. Seaplanes can only take off and land on water with little or no wave action and, like other aircraft, have trouble in extreme weather.