1. They can smell fear.
Babies, like horses, can sense when their keepers are afraid of them (and other things, such as blankets coming alive and sneaking across the floor to smother your baby). With your firstborn, everything is a new experience, and everything is fraught with potential danger. You make yourself crazy, and you probably make your baby crazy, too. When the second one comes and he is an "easy" baby, you may be tempted to believe that he is a gift for surviving the first baby, but it's probably more likely that you're a bit more chilled out about things.
2. They know when they do not have your full attention.
Have you ever been nursing or rocking a baby with your mind overflowing with all of the things you are going to do as soon as you get.this.darn.baby.to.finally.go.to.sleep? Have you noticed that on these nights, bedtime takes even longer? My theory is that they have to receive X amount of attention before sleep can be achieved, and the best and easiest way to reach this mark is to focus on the baby at hand. Do not allow yourself to be distracted by how you're going to eat a piece of cake while standing in front of the fridge and then will be pouring yourself a glass of wine and finishing the movie you keep falling asleep through. Focus on that baby. Send him happy thoughts and all your attention. You'll get out of there faster.
3. Two words: Murphy's Law.
Murphy's Law as it applies to babies goes something like: If you forgot to pack an extra outfit, your baby will produce a giant, outfit-ruining poop. See also: If you burp a baby without a burp cloth handy, you'll be spewed upon (conversely, if you do use a burp cloth, there will be no spit-up). This especially applies when you are wearing clothes that are appropriate for wearing out of the house.
4. They have impeccable timing.
They will stop screaming in their car seats right before you reach your destination. They will take a long, blissful naps on the days you have to be somewhere at a certain time. When you tell another person (out loud or in writing) that your baby has been sleeping for an hour and OHMYGOSH how awesome is this?... your baby will wake up. (Tip: If one of these epic naps happens and you need to get to that birthday party, proclaim loudly and often that your baby is taking a long, blissful nap.) And, of course, they know when, ahem, adult activities are happening and interrupt accordingly.
5. It is hard to be a baby (and it eventually gets to be less hard).
Oh, when babies are tiny, they want you to know: THIS IS THE MOST MISERABLE I HAVE EVER BEEN. MY LIFE IS HORRIBLE. Because, you're, say, doing them a favor and changing their dirty diaper. Sometimes they get mad at boobs: The milk is coming too fast, the milk is coming too slow, the nipple looked at me funny, Mercury is retrograde, I have to burp, I am tired, I am NOT tired, it is growth spurt time, I like to hear myself cry. The vast majority of the time, the answer to "Why is my baby crying?" is "Because it is hard to be a baby." It has nothing to do with you (or your milk). Don't take it personally, or try not to, anyway. It'll pass. Tomorrow you may not have to deal with a baby shrieking while you're trying to feed her because the growth spurt will have passed. A couple of months from now you'll stop and notice (not out loud; see #4) that no one has cried for half of a day. And then you'll probably cry because your baby is all grown up and practically in college.
