Here's why:

* Grew in a cave: Stalagmites form inside caves.

* Not alive: They are not living organisms, but geological formations.

* Began water and minerals: They are formed by dripping water carrying minerals.

* Now solid: Over time, the minerals harden and form a solid structure.

* Never see the sky: They are hidden deep within caves.

Let me know if you want to try another riddle!