Why does an ice cube float in milk?

For the same reason it floats in water and other water-based liquids: the ice is less dense than the liquid. When water freezes, the hydrogen bonds force the oxygen atoms farther apart, and this "lattice form" takes up more space than the liquid molecules. So the ice takes up more space for its mass, is less dense, and will float on the surface of water...or milk. The denser the liquid, the higher the ice cube will sit above the top surface.