Here's why:

* Density: Metals are generally dense, meaning they pack a lot of mass into a small space. This is why most metals sink.

* Lithium's Low Density: Lithium is the lightest metal, with a density of just 0.534 g/cm³. This is less than the density of water (1 g/cm³), making it float.

Let me know if you'd like to learn more about metals or density!