Here's why:

* Buoyant force acts upwards, pushing an object upwards and making it feel lighter. It's the force exerted by a fluid (liquid or gas) that opposes the weight of an object immersed in it.

* Gravity acts downwards, pulling an object towards the center of the Earth. It's the force that gives objects weight.

These forces work in opposition to each other. If the buoyant force is greater than the force of gravity acting on an object, the object will float. If gravity is stronger, the object will sink.