You probably know that breastmilk contains antibodies. You probably also know that it can contain antibodies tailored to germs the baby is exposed to because the mother makes antibodies against germs she's been exposed to, and since moms and babies spend so much time together, these are typically the same germs their babies are exposed to. But what happens if your baby is exposed to germs you haven't been? These germs, if they're going to make your baby sick, would be entering through the mouth or nose. So the rational thing to do, of course, is to ingest a sampling of the germs on your baby's face so your body can start to make antibodies to those germs! Or maybe it doesn't seem that rational, which is why the instinct is there. So all you have to think is "Awww, you're so cute. I'm gonna kiss your cute little nose!" Betcha didn't know that kiss could be protecting your child from illness!