At what point does an atom cross an international border, and can it exist on the cusp, not in either country? I would say that the border, not being a material object as such, is infinitesimally thin. So the atom can't possibly be as thin as the border, therefore there will be a time at which the atom can be said to be on one side of the border, straddling the border at another time, and finally on the other side of the border. The atom can be straddling the border partially, exactly on the border with half on one side and half on the other, or fully on one side or the other. No matter what, then, some part of the atom is always in a country. These are the conclusions I can draw from my limited knowledge of such things. Thinking about this answer has raised more questions than I had before. This usually happens when I arrive at any answer. I assume that a border be infinitesimally thin, but is it that true, or even possible? Is it pointless to talk of anything that exists being thinner than the thinnest thing possible, i.e. a quark? (Is a quark the thinnest thing possible?) If so, are borders one quark thick? In which case, while an atom is going to be in one of the positions I mentioned above (straddling, 50/50 or completely over the border) a single quark could exist exactly on the border and not be in either country, right? And what about the uncertainty principle - wouldn't this make it impossible to determine the exact position of the quark anyway? And if quarks are pointlike, does this mean they have no width at all?