While you might think of Mercury as the fastest due to its proximity to the Sun, Jupiter actually has the shortest day in terms of its rotation period.

Here's why:

* Jupiter rotates incredibly fast: It completes a full rotation on its axis in just under 10 hours. This means a day on Jupiter is significantly shorter than a day on Earth (24 hours).

* Mercury's rotation is slow: Despite being close to the Sun, Mercury rotates very slowly, completing one rotation in about 59 Earth days.

So, even though Mercury is the closest to the Sun, Jupiter's rapid rotation makes it the inner planet with the shortest day.