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Build a Better Bag Boat

Density and buoyancy have an inverse relationship. As an object’s density increases, its buoyancy decreases. And as an object’s density decreases, its buoyancy increases.

A great example of this inverse relationship is ice floating in water. When water freezes, it increases in volume as the water molecules move further apart to accommodate the lattice structure of ice. Because it’s now less dense than water, it floats.

We can use density to explain why ships float even though they are made of steel. A solid piece of steel would sink, but a ship is built in such a way that it encloses large amounts of open space. This means that the overall density of the ship is less than that of water, and it floats. 

This activity motivates students to explore the relationship between density and buoyancy by designing a boat.

Challenge students to modify a piece of paper by folding and cutting and adding other materials. The basic design of a paper boat can be something as simple as cutting the top off the paper bag or as complex as folding the bag into an origami boat.



  • Demonstrate an understanding of the relationship between density and buoyancy by building a boat.


  • Per Class:
    wax paper
    aluminum foil
    plastic wrap
    masking tape
    plastic sandwich bags
    1 large tub of water
    coins, dried beans or other “cargo”

  • Per Group:
    1 paper lunch bag
    1 “passenger” (small piece of coloured construction paper)
    25 tokens

Key Questions

  • What is/was your strategy when you designed and built your boat?
  • What were some of the choices you made about your design, and why?
  • Why were some boat designs less successful?
  • Why were some boat designs more successful?
  • What other materials would you have liked to use?

What To Do

Set up a “store” with materials from the Supply Pricing List. Post the list so that the students can see it.

Supply Pricing List

1 straw 1 token
5 cm piece of tape 1 token
5 cm x 5 cm sheet of wax paper 5 tokens
5 cm x 5 cm sheet of saran wrap 10 tokens
5 cm x 5 cm sheet of aluminum foil 15 tokens
Plastic sandwich bag 20 tokens


  1. Divide the students into groups of 2 or 3 and challenge them to make a paper bag into a boat. They should use their understanding of buoyancy and density to help them.
  2. Explain that the idea of the competition is to buy the materials needed to build a boat that will hold a lot of cargo, while keeping their passenger safe and dry. The winning team is the team whose boat holds the most pennies and has the most pennies left over from their supply budget to add to the total.


  1. Give each group a paper bag, a “passenger” (small piece of coloured construction paper) and 25 tokens.
  2. Give the students 15–25 min or so to plan their strategy, purchase materials, and create and test their boats.
  3. Gather the boats for their maiden voyage. Test each one by placing the “passenger” in the boat, floating it in the water, and dropping in the “cargo” (coins or other weights) one by one. The winning team is the group whose boat can hold the most cargo while keeping its passenger safe and dry.


  • Go back to the workstation, modify the boat, and try the experiment again.