Submarines are large vessels that move underwater. The windows of submarines, called portholes, are usually round. Submarines have a tool called a periscope that sticks out above the water.

When a submarine dives, its ballast tanks are filled with water, which makes the submarine heavier.

When it’s time to resurface, the water is replaced with air, which makes it lighter and the submarine rises.

• Old Water Bottles

• Sand or Rocks (optional)

• Bin of Water

• Experiment with putting an uncapped bottle in the water. What happens when it fills with water?

• What happens when a capped bottle is placed in water?

• What happens if a capped bottle has sand, rocks, or water inside?

When we initially put the empty bottles into the water, they both floated on the surface, but as they filled them with water, they sank to the bottom of the bin.