Plastic are made of one type material. why are they not classified as a composite

It's because plastic is not a composite, it's actually a compound. Composites are actually a mixture of different types of materials.

The density of water is 1 g/cm cubed and in order for the pebble to sink its density has to be greater than the density of water and in order for the twig to float its density has to be less than the density of water