When I was a kid, I remember being really astounded by the fact that arithmetic is consistent. Like uh, 2 + 3 + 4 is 9. And 9 - 4 is 5, which is 2 + 3, and 9 - 2 is 7, which is 3 + 4! And 7 is also 6 + 1, and guess what, 9 is also 6 + 1 + 2, and even more amazingly, 6 is 4 + 2, and (remember 9 - 4 is 5) 2 + 2 + 1 is 5! No matter what you do it's always consistent, you can't trick it by adding and subtracting some convoluted sequence of numbers until it gives you an answer that's inconsistent with the rest.
When I was first learning arithmetic I remember that I just found this crazy, like, how does it always work out? That shouldn't be possible. And I figured that, naturally, when I got older, someone would eventually give me the answer to this incredibly pressing question. And, well, it turns out nobody has the answer, and they actually proved that it's impossible for anyone to give a satisfying answer. So, that's a bit disappointing.