"Well, let's suppose it takes three years for a little angel to become interesting," Isabella says, rocking a sleepy Keziah in her arms. "And let's say I'm only going to have two, that Keziah's going to be the last one. So when she's three, she'll be interesting, and if she could be interesting when she's two instead, then all my little-angel-interestingness would be done faster. But what if I'm going to have six of them? Then by the time the last one was interesting, you would be at least ten years old, if I had them one at a time. If they all were born at the same time then the same amount of little-angel-interestingness would be done faster, even if it still took them all three years."