Junichi from Japan: Pluto's moon, Charon, is extraordinarily large as compared to Earth and Earth's moon.
How do you suppose Pluto's moon, Charon, was formed?
Well, it turns out that, in fact, Pluto and Charon have a lot more in common with the Earth/moon system than Junichi might think.
They probably both formed from a giant cosmic collision, two objects colliding with each other about 4.6 billion years ago.
In the one case, in the Earth/moon system, we had the Earth and a Mars-sized object that collided with each other,
eventually separated into the Earth and the moon.
In the case of Pluto, what we think happened was two Kuiper Belt objects -- these -- they, probably Pluto-sized,
approximately, smashed into each other and created two roughly equal-sized objects.