Explanation:

A steel ball floats on mercury because the density of steel (approximately $7.85 \text{ g/cm}^3$) is significantly less than the density of mercury (approximately $13.6 \text{ g/cm}^3$). According to Archimedes' principle, an object floats if its density is lower than the fluid it displaces. It would sink if its density were higher. Steel does not dissolve in mercury, nor does it suspend in the middle unless its density perfectly matches the fluid.