Buoyancy with Lego Boats

SCIENCE – Teach students about buoyancy by conducting a sink or float experiment, then use engineering skills to build a Lego boat and try to make it float.

Learning Objectives:

With Buoyancy with Lego Boats, students will learn to experiment and test, make predictions, sort and categorize objects, observe and describe what happens, record data using graphs or charts.

Materials needed:

  • Random objects for students to test if they sink or float
  • A container filled with water
  • Download the student chart to track their designs and progress
  • Assorted Legos for students to build their boats
  • Pennies to test the density

Begin by explaining the following scientific principles:

Objects that float are positively buoyant and there are several reasons why they float. Objects that are less dense than water will float. Density refers to how close together the molecules of an object are. For example, most rocks sink in water because they are denser than water. Also, the surface (i.e., the outside of an object) that touches the water displaces it, or pushes it out of the way.

The shape of an object also affects how water moves around the object’s surface. For example, the shape of a boat creates a large surface for water to push against. However, if too much weight is added to a boat, it will sink beneath the water. (Students will test this after they construct their Lego boat by adding pennies.)

Some objects are neutrally buoyant. This means that they sink beneath the water’s surface, but they do not sink all the way to the bottom. This happens when an object’s density is the same as the density of the water it is in.


Start by explaining that you will be playing a game called sink or float.

Students will have 10 seconds to choose an item from the room and bring it to you, then set a timer or count to 10 while the children choose their items.

As a group, sort the items into a “sink” pile and a “float” pile, then test the items in a container of water to see if the predictions were correct.

You might also consider asking questions like:

  • What are the characteristics or features of objects that float?
  • What are the characteristics or features of objects that sink?
  • What would happen if you place an object that sinks on top of an object that floats?


After learning about buoyancy, students will sketch a design of their boat and then construct it from Legos. Here the experiment was done with a first grader and pre-K at home in the wake of school closures because of the Coronavirus.

Have students discuss what makes a boat float and come up with an initial design. Depending on the age, students can simply draw their plans or for older students, have them write why they chose a specific design. 

Next, have them construct their boats from legos.

Test whether their boats sink or float.

Have students fill out a chart to track their attempts at buoyancy with Lego boats, then have them rework their design to try again. Once the students have a boat that floats, you can attempt to add pennies to see how much weight the boat will hold before it sinks (use our Aluminum Foil Boat lesson for reference).

Additional Resources

Underwater Exploration: A National Geographic website that provides information on the many expeditions and discoveries made by National Geographic explorers. Includes articles, videos, and other multimedia sources.

Buoyancy: A quick overview of how things float and key buoyancy concepts.

Looking for something for older students? Check out this Museum of Innovation Lesson Plan on Buoyancy for students in grades 3-8.

Education Standards


ENGINEERING: K • K-2-ETS1-1 • K-2-ETS1-2 • K-2-ETS1-1