Here's why:

* Mercury: It has the fastest rotation period of all the inner planets, spinning once every 59 Earth days.

* Venus: Despite being the slowest moving planet in our solar system, it actually rotates very slowly in the opposite direction of most other planets, taking about 243 Earth days to complete one spin.

* Earth: We all know it takes about 24 hours for Earth to rotate once.

* Mars: Mars takes about 24.6 hours to complete one rotation.

Therefore, Mercury spins the fastest among the inner planets.