Despite how readily we poke fun at the answers given on the linked page, I found it interesting that I had to drill down this far to find a satisfactory answer.

Despite how readily we poke fun at the answers given on the linked page, I found it interesting that I had to drill down this far to find a satisfactory answer. I liked the explanation given by my high school physics teacher: in the end, it boils down to frame of reference -- since both the moon and the pen are in orbit (aka perpetual free-fall), the earth's gravity basically becomes "background" to the situation and has no effect on the relationship between the pen and the moon. It's not the concept of gravity, the mathematical formulas, or anything else that's really hard to grasp. Looking at things from the right frame of reference is hard, and not just in physics either. I struggle with this in my daily work, even for simple things like getting widgets to align correctly relative to each other.