Why cork floats in water?

Any object placed in water will be pulled down into the liquid by gravity. But an object less dense than water will only be pulled down until the object displaces an amount of water equal to its own mass. Then it will float. The water will be pushing up equal to the force of gravity pulling down - an equilibrium. Any object that weighs less than its own volume of water will float. It's lower overall density will result in buoyancy.