Archimedes principal

Archimedes' Principle states that a body submerged in a fluid experiences an upward buoyant force equal to the weight of the fluid it displaces.

In simple terms:

When you place an object in water (or any fluid), it pushes some of the fluid out of the way (displaces it).

The fluid, in turn, pushes back on the object with a force equal to the weight of the displaced fluid.

If the buoyant force is greater than or equal to the object's weight, it floats; if it's less, it sinks.

This principle explains why ships float, why some objects sink, and how submarines control their depth.