Realizing you've memorized "The Wheels on the Bus"
Including the hand motions! No parent thinks it will happen to her. And then it does: The baby fusses, and there's only one way to calm her. You start singing -- in that special voice -- at the post office or in the video store. Congratulations! You've put your child's needs ahead of your own self-consciousness. (You've also learned what works to calm your crier, be it song or tickle games.)
Now you've crossed to the other side -- parents only. You may not know it when you're hitting these milestones, but you'll realize it when you automatically turn around when a nearby baby cries, or when you introduce yourself as "Sophie's mom." Being a parent is rewarding, frustrating, and challenging. It's also a work in progress. And you're doing it.