Here's why:

* Density: Density is a measure of how much mass is packed into a given volume. A denser object has more mass squeezed into the same space.

* Buoyancy: When an object is submerged in a fluid (like water), it experiences an upward force called buoyancy. This force is equal to the weight of the fluid displaced by the object.

* Floating vs. Sinking: An object will float if the buoyant force acting on it is greater than or equal to its weight. An object will sink if its weight is greater than the buoyant force.

How Density Determines Floating:

* Less Dense Object: A less dense object displaces a smaller amount of fluid than a denser object of the same size. This means the buoyant force on the less dense object is smaller. However, because the object is less dense, its weight is also smaller. This allows the buoyant force to be strong enough to keep it afloat.

* More Dense Object: A denser object displaces more fluid, creating a larger buoyant force. However, its weight is also greater due to its higher density. This results in the weight overpowering the buoyant force, causing the object to sink.

Example:

* A wooden block (less dense) floats on water (denser) because the buoyant force from the displaced water is greater than the weight of the wooden block.

* A rock (denser) sinks in water because its weight is greater than the buoyant force from the displaced water.