How do submarines sink and float?
For grown-ups: the science
Submarines fill special tanks with water to sink and use air to push the water out to float up.
Submarines control buoyancy with ballast tanks. Flooding tanks increases the submarine's average density so its weight exceeds the buoyant force; compressed air expels water, lowers average density, and allows it to rise. Diving planes and propulsion help control depth and angle.
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Transcript
A submarine glides under the ocean waves. How can such a big boat sink and float?
A submarine has big tanks that can fill with water or air. These tanks help change how heavy the submarine is for its size.
To sink, it lets water into the tanks. Then it is heavier than the water it pushes aside, so it goes down.
To rise, it blows air into the tanks and pushes water out. Then it is lighter than the water it pushes aside, so it floats up.
So submarines use water and air in special tanks to travel down and back up.