Here's why:

* Reactivity Series: Metals are ranked in order of their reactivity in a series called the reactivity series. The most reactive metal is at the top, and the least reactive is at the bottom.

* Position: Calcium sits below potassium (K) but above sodium (Na) in the reactivity series.

* Reaction Characteristics: Calcium reacts readily with water, oxygen, and acids. It's used in various applications, including alloys and in the production of fertilizers.

While francium (Fr) is considered the most reactive, it's incredibly rare and radioactive. Calcium is the second most reactive metal in a practical sense.