Personally I would go for stellar astrophysics. Not because it's the most practical, but because like quantum physics, the lack of being able to really comprehend the large scale (quantum being the opposite, as you very well know) will keep your mind occupied for hours outside of course content...
Taking this into consideration, if you exclude a force strong enough to do that, I believe you're left with an ever so slightly different answer depending on the objects being in relatively high external pressure (gas atmosphere maybe) or if the objects are in a vacuum.
There are so many variables across such a huge timeline, it's impossible to give a definitive answer. What if intelligent life is evolving and destroying billions of other planets and it's just a common trend in the universe? What if we're all in the Matrix and AI is the dominant "life" form...
Why objects stay together has been explained, but to your question of "removing a single molecule"
http://learn.genetics.utah.edu/content/begin/cells/scale/
Zooming in will show you both a skin cell and a carbon atom. I don't think I'll ever be able to grasp the scale of the molecular level.
You have to find something that she is equally passionate for and compare. She doesn't have to understand, she just needs to relate.
I did this with my mother on multiple occasions. One time was a while ago, after failed attempts to explain why math is beautiful because it is TRULY perfect. She...