Sink or Float
Background Information & Activities

When an object floats, it can stay on the surface of a liquid by itself. Different solids, liquids, and gases can float. For example, both cork and oil will both float on the surface of water. Some gases, such as helium, can rise, or “float” in the air. When an object sinks, it moves down with gravity. Different solids, liquids, and gases sink. For example, a solid coin will sink in a cup of water. Gases that are heavier than air, such as propane and butane, can sink as well.

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. Gravity pulls the object down, but the difference in pressure above and below the object causes an upward force. 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 pushes away, and an object will float if it weighs less than the water it pushes away. 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 displaces more water. A canoe shape can push 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.

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 your children are more advanced and ready to explore sinking and floating further, we recommend screening BrainPOP’s Buoyancy movie.

Sink or Float Teacher Activities – Click Here!

Sink or Float Family Activities – Click Here!

Sink or Float Teacher Activities

Sinking Soda

Bring in cans of regular soda and diet soda and have students predict which will sink and float. Have students explain their predictions. Then drop both cans in a large bowl or tank of water. Have students make and record their observations. Why does the diet soda float? Why does the other soda sink? Have students research the Internet and determine if their predictions were correct. Encourage them to draw conclusions and share their ideas with the class.

Sunken Treasure

Give small groups a bowl of water and a set of waterproof objects such as metal and plastic spoons, toothpicks, bolts, screws, number cubes, small balls, and metal keys. Have students predict which items will float and which will sink. Then have groups discuss their predictions and drop different items into the bowl. Students should sort the floating items from the sinking items. After everyone has completed the experiment, place all the sinking objects into a tank or large pot filled with water. Hide the “sunken treasure” in your classroom or, if possible, the school and draw a map that helps the students find the treasure.

Water Line

Give pairs of students a clear cup of water and small items that sink or float. Remind them that when an object is placed in water, the level will rise. Have them predict how much they think the water will rise for each item. Students should mark the original water level and draw their predictions on the side of the cup. They can use different colors of marker to distinguish their own predictions. Then have them drop different items into the cup, make observations, and record their data. Encourage them to measure with a ruler to see how much the water rose.

Sink the Ship

Float a toy boat in a clear tub filled with water. Have students find ways to sink the ship in the tank. Students may wish to add items to weigh the ship down, so it is important that they use items that are waterproof. Explain that you can waterlog the ship by tipping it over and filling it with water. Have students predict why the boat sinks when it tips over. Explain that air inside the ship helps the boat float, but when it tips over, the air escapes. Thus, the boat sinks.

Sink or Float Family Activities

Boat Float

Have your child make boats out of different materials, such as toothpicks, cork, paper, craft sticks, fabric, and clay. Have him or her make predictions about which boat will sink or float and write them down. Then have him or her try out their boats in a bathtub. Which boat floats? Which boat sinks? Encourage your child to explain what happened. Then have him or her think of ways to make a sinking boat float and vice versa. Allow your child to experiment using different shapes and adding weight to the boats.

Floating Animals

Together go to a local zoo and find animals that float. Animals like ducks, seagulls, beavers, and otters can all float on top of the water and walk on land. Have your child draw comparisons between the animals and write down their observations. Then have your child research on the Internet or at the library about his or her favorite floating animals. What body parts help them float and swim? How long can they hold their breath? How do they stay beneath the surface of the water? How much time do they spend in the water? Encourage your child to think of adaptations the animals have that allow them to live in the water.