Why Does Mercury Look So Much Like the Moon?

(from here)

They are cosmological twins!

The two bodies are even fairly close in size.

I thought Mercury was supposed to be covered with molten rock on the surface from being close to the sun (what I learned in school a long time ago). But the craters here are identical to the Moon craters? and why are the craters always perfectly round-- implying a straight vertical impact?

Wiki article on Mercury.

Possible explanations for the similarity between Mercury and the Moon:
1) NASA is faking the photos
2) Mercury and the Moon were constructed by the same beings
3) Mercury and the Moon had similar physical origins and exposure to asteroids-- and somehow ended up in very different places in the Solar system