Limitless?

Bug: Mom, I don't get the saying "The sky's the limit". I mean, what if I want to go higher than the sky???

Me: Well, the sky doesn't really have an ending. It just goes on forever, like into space and then further. So the sky doesn't have a limit and neither do you! You can just go as high as you want!

Bug: Well, until you die.

Me: Huh?

Bug: Well, after you leave Earth's atmosphere you can't breathe and eventually you'll die. So the sky does have a limit. A limited supply of oxygen.