Experience

When you are a new parent for the first time, you have absolutely no clue what you're doing. Or at least I didn't. So you buy parenting books, subscribe to parenting magazines, and watch shows that help navigate the new terrain you've found yourself engulfed in. You can no longer pass by any type of information kid related. Anything to get your baby to a. stop crying, b. sleep through the night, and c. do something that makes him better than all the other kids you know. You learn 20 different ways to sleep train, the best order of solid food introduction, milestones (otherwise known as if-your-child-isn't-doing-these-things-we-need-to-discuss-his-need-for-the-short bus), and all the toys your child needs to be valedictorian of his preschool class. After a while, the books gather dust, subscriptions aren't renewed, and you find yourself watching trashy tv shows again, confident in your now amazing skills as a parent.
Another baby arrives, and you give yourself a quick refresher course, along with buying 3 potty-training books.
Another one comes, and you remind yourself to find the books.
Suddenly you find yourself the parent of four, so caught up in life you realize you've forgotten all the at-one-time-invaluable information you learned years ago. Your baby is a terrible sleeper because you didn't train her properly, she cries unless you're holding her because you've never done floor time, you can't remember when you're supposed to start solids, and when the doctor asks about milestones, you draw a blank (She is smiling right?)
You realize that this last child will have hand-me-down parents, parents who were new and fresh for their first born, but are now like your sister's old shirt, faded and worn. But you also realize that with all the washings that shirt has been through, it's gotten softer, more comfortable, and more experience than you would care to think about.