Here's why:

* Gravity: Pulls objects downwards towards the center of the Earth.

* Buoyancy: An upward force exerted by a fluid (liquid or gas) that opposes the weight of an immersed object.

When an object is submerged in a fluid, the fluid exerts an upward force on the object, which is called buoyant force. This force is equal to the weight of the fluid displaced by the object. If the buoyant force is greater than the object's weight, the object floats. If the buoyant force is less than the object's weight, the object sinks.

Therefore, buoyancy and gravity are opposing forces that determine whether an object floats or sinks.