Sink or Float?

What You Need


  • Who Sank the Boat? by Pamela Allen
  • A large tub of water about 1/2 full for the teacher demonstration 
  • A bucket, or bowl of water about 1/2 full for each group
  • At least 6 different items for each group that are made of a variety of different materials
Sink or Float?


To make and test predictions about sinking and floating and then classify objects according to whether they sink or float.


In this activity students will determine whether various objects sink or float in water. Whether an object sinks or float in a liquid depends mainly on two factors: density and buoyancy. However, at this level, students do not need to explain why objects sink or float. They are rather to be encouraged to observe that the same objects will sink or float every time, i.e., that there is consistency in the way the objects behave. This will help students devise their own ideas about physical properties and how they can be used to describe and categorize objects. 

This lesson will also provide practice categorizing a variety of objects according to observable characteristics.

Planning Ahead

Suggested materials for activity include: wood, metal, plastic, aluminum foil, apples, oranges, plastic bottles, toy blocks, paper, bathtub toys, plastic forks, rubber balls, soda-bottle caps, pencils, erasers, and sponges.


Read aloud Who Sank the Boat? by Pamela Allen. This book (about a cow, donkey, sheep, pig, and mouse who decide to go for a boat ride) can be used as a springboard for discussing sinking and floating. See Carol Hurst's Children's Literature Site  for more ideas for using this book.

If you don't have the book, you can brainstorm on what it means to sink or float. Ask students to describe things that they have seen sink or float. Elicit student ideas on what kinds of things they think will sink or float, for example: Can people float? 

Then, do the Float and Sink interactive activity on the BBC Schools website. Once the page loads, you can simply follow the arrows to go through the activity. Once you get to the screen where students are asked to decide which objects float, ask students to guess which objects will float or sink. On the next screen, students should try to decide why all of the objects float.


Discuss the results of the Float and Sink activity, then continue with the following activity that will help students develop their ideas about the physical properties of matter. The ideas in this lesson will help lay the foundation for exploration of concepts such as density and forces in the later elementary grades.

In this activity, students will have the opportunity to experiment and explore with different types of objects, and to record in some form the results of their experimentation.

Set the large tub of water in the front of the class and have students gather around you. Examine one of the objects closely, asking students to note some of the things they observe about the object. Then ask students to predict whether they think it will sink or float. Place the object in the water and ask the students to describe what happens. Repeat this procedure with several items.

Next, divide the class into groups of four or five. Give each group a bucket or bowl of water, an assortment of items to test, and a Sink or Float Activity Sheet.

For each item, have students:

  1. Write or draw the item in column 1. (You may want to do this for younger students before you duplicate the test sheet.)
  2. Predict whether it will sink or float and record their prediction in column 2.
  3. Place the item in the water and observe what happens.
  4. Record their results in column 3.
  5. Repeat the procedure and record the results in column 4.
  6. Place the items that sank in one pile and the items that floated in another pile.

After each group has finished testing their objects discuss the results using the following questions:

  • How many of your predictions were correct?
  • Did your predictions get better, worse, or stay the same?
  • Look at the pile of objects that sank. Describe them. Do they have anything in common with one another?
  • Look at the pile of objects that floated. Describe them. Do they have anything in common with one another?
  • Compare the results for each group. Did everybody get the same results? If any of the results were different, ask students to replicate their trial.

Building upon this exploration, children should discuss similarities and differences in the material characteristics of objects that they think affect whether they float or sink (i.e. objects made of wood will usually float; objects made of metal will usually sink). Children may come to the conclusion that heavier objects generally tend to sink in water. However, make sure that children understand that weight is not the only factor.

As they continue to investigate floating and sinking, they should be building the understanding that objects float because a force equal to the weight of the water they displace buoys them up. In later grades students can build a more complete understanding of these phenomena by measuring mass, displacing volumes, and calculating densities.

Children might also want to explore questions such as these: Does it matter how deep the water is? Does it matter how much water there is? Have the students suggest different things to try and give them an opportunity to test their ideas.


Assess the lesson by bringing in a variety of toys that can be placed in water, such as rubber balls of different weights, frisbees, toy boats, and so on. Ask students, "Which of these toys would sink in a swimming pool and which would float?" 

Have children draw the toys that would sink and those that would float in the appropriate columns on the Sink or Float Assessment Sheet.


For more advanced students, you can extend the ideas in this lesson by having students explore the question, "Can we change something from a sinker to a floater?"  Children can try making boats from wood, polystyrene, or clay. Encourage them to experiment with different shapes or to make sails for the boats.

Additional ideas about sinking and floating can be found in this resource:

  • Making Things Float & Sink/With Easy-To-Make Scientific Projects, by Gary Gibson, Copper Beech Books; ISBN: 156294617X ; 1995

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Lesson Details

Grades Themes Type Project 2061 Benchmarks National Science Standards