Here's why:

* Periodic Table Position: Potassium (K) is in Group 1 (alkali metals), while calcium (Ca) is in Group 2 (alkaline earth metals). They sit directly next to each other on the periodic table.

* Similar Electron Configuration: Potassium and calcium have similar electron configurations in their outer shells. Potassium has one electron in its outermost shell, while calcium has two.

* Chemical Properties: While they are in different groups, they share some similarities in their chemical reactivity. Both react vigorously with water, though calcium's reaction is less vigorous than potassium's.

Let me know if you have any other questions!