Because she did not look behind, September did not see the smoky-glass casket close itself primly up again. She did not see it bend in half until it cracked, and Death hop up again, quite well, quite awake, and quite small once more. She certainly did not see Death stand on her tiptoes and blow a kiss after her, a kiss that rushed through all the frosted leaves of the autumnal forest, but could not quite catch a child running as fast as she could. As all mothers know, children travel faster than kisses. The speed of kisses is, in fact, what Doctor Fallow would call a cosmic constant. The speed of children has no limits.
When you are born...your courage is new and clean. You are brave enough for anything: crawling off of staircases, saying your first words without fearing that someone will think you are foolish, putting strange things in your mouth. But as you get older, your courage attracts gunk and crusty things and dirt and fear and knowing how bad things can get and what pain feels like. By the time you're half-grown, your courage barely moves at all, it's so grunged up with living. So every once in a while, you have to scrub it up and get the works going or else you'll never be brave again. Unfortunately, there are not so many facilities in your world that provide the kind of services we do. So most people go around with grimy machinery, when all it would take is a bit of spit and polish to make them paladins once more, bold knights and true.
All children are heartless. They have not grown a heart yet, which is why they can climb tall trees and say shocking things and leap so very high that grown-up hearts flutter in terror. Hearts weigh quite a lot. That is why it takes so long to grow one. But, as in their reading and arithmetic and drawing, different children proceed at different speeds. (It is well known that reading quickens the growth of a heart like nothing else.) Some small ones are terrible and fey, Utterly Heartless. Some are dear and sweet and Hardly Heartless at all.