A pontoon is a flat-bottomed boat or a hollow, airtight float used to support something on water. The most common type you'll see is a pontoon boat, which has two long metal tubes (the pontoons) running along each side, with a flat deck built on top. These boats are popular on lakes because they're stable, roomy, and hard to tip over.

Pontoons can also be temporary floating platforms. Engineers use pontoons to build pontoon bridges, which are bridges that rest on floating supports instead of pillars sunk into the riverbed. Military forces have used these for centuries to quickly cross rivers when regular bridges aren't available. During World War II, engineers could assemble a pontoon bridge in just hours, allowing tanks and troops to cross rivers that would otherwise stop their advance.

What makes pontoons special is their buoyancy: they displace enough water to support heavy weights while staying afloat. Some seaplanes use pontoons instead of wheels, letting them land on lakes and rivers. The key principle is always the same: spreading weight across floating supports so they stay on top of the water.