Here's why:

* Buoyancy: Objects float because of buoyancy. Buoyancy is an upward force exerted by a fluid (like water) that opposes the weight of an object submerged in it.

* Archimedes' Principle: This principle states that the buoyant force on an object submerged in a fluid is equal to the weight of the fluid displaced by the object.

* Density and Floating: For an object to float, the buoyant force must be greater than or equal to the object's weight. Since the buoyant force is determined by the weight of the fluid displaced, if the object's density is less than the fluid's density, it will displace enough fluid to equal its own weight and float.

In summary: A block floating in water means it's less dense than water.