Not only to 4th graders. I have a PhD in physics and the more I think about that, the less obvious it becomes (or at least makes you seriously think about it).

Not only to 4th graders. I have a PhD in physics and the more I think about that, the less obvious it becomes (or at least makes you seriously think about it). The average person is probably in the easiest situation because they learned how to add fractions without further philosophy and they can live happily after.