>> "The gravity of the moon can be said to be negligible, and also the moon's a vacuum, there is no external force on the pen. Therefore it will float where it is."
That one's almost correct; it's just an issue of scale; they don't realize how big/heavy the moon actually is.
The issue is that they presumably understand that the moons gravity isn't negligible, since damn near everybody on earth has seen the Apollo 11 moon landing videos. They are not merely missing information needed to answer the question and making the wrong assumption. They have all of the necessary information, but get it wrong anyway because for whatever reason, they aren't using all of the information they have.
I would say that the footage has been used enough in various forms of media to have just about everyone covered. Stuff like this at least: https://www.youtube.com/watch?v=182oUgBfoLE
One way or another, I think everyone but very small children or reclusive hermits have seen at least some footage of an Apollo astronaut bouncing around on the moon.
That one's almost correct; it's just an issue of scale; they don't realize how big/heavy the moon actually is.