Why do some objects float why others sink

Because so objects are denser than water and some are less dense than water

It will sink. This means whether or not an object will float or sink depends on its own density and the density of the liquid it is placed in. In the case of water, an object with a density less than 1 g/cm3 will float. The closer its density is to 1 g/cm3, the more of it will sit below the water level.