This page provides information to support educators and families in teaching K-3 students about sinking and floating. It is designed to complement the Sink or Float topic page on BrainPOP Jr.

In this movie, children will learn that when an object floats, it can stay on the surface of a liquid by itself. When an object sinks, it moves down with gravity. Children will explore how some objects push water out of the way in order to float and how shape can help things float. We recommend doing plenty of hands-on activities and experiments with children.

Displacement explains why objects sink or float. Displacement occurs when you place something in a fluid, or any substance that flows, and it moves the fluid out of its way. You can watch displacement at work when you drop an object in a cup of water and the water level rises. The object pushes the water out of its way, making the water rise. An object will sink if it weighs more than the water it displaces, and an object will float if it weighs less than the water it displaces. The Greek mathematician Archimedes discovered that the amount of water displaced by an object depends on the mass of that object. Mass is the amount of matter in a substance, and dense objects have more mass than less dense objects. Dense objects that do not displace much water will sink, while less dense objects that displace a lot of water will float.

Shape can also help an object float. A ball of clay will sink, but a canoe shape made from the same amount of clay can float because it pushes more fluid out of its way in relation to its weight. The amount of air inside of an object can also help it float. Boats can float despite the heavy and dense materials used to build them because of the large amount of air inside the hull. Hollow objects, such as table tennis balls or an empty plastic bottle, are able to float better than solid objects like baseballs or filled water bottles.

Many children believe that heavy objects sink and lighter objects float. Point out that some heavy objects like boats float, while relatively lighter objects such as coins sink. Encourage them to hypothesize why objects sink and float. Have them draw on prior knowledge and describe observations they have made of sinking and floating objects. Encourage them to make comparisons and connections.

Most children can easily observe and recognize objects that sink or float, but understanding buoyancy at a scientific level may be difficult for young children. We recommend watching the Sink or Float movie more than once, doing demonstrations or experiments, completing the activities and features together, and leading class discussions to reinforce the ideas in the movie. If children are ready to explore sinking and floating further, we recommend screening BrainPOP’s Buoyancy movie.