How does one teach a people-loving three year old that it is not okay to open the door when someone knocks, that he needs to wait for Mom or Dad to see who is at the door???
He doesn't understand that bad people exist and it's just not safe to open the door to a stranger, especially when he's by himself in the room. And, being the independent little bugger that he is, telling him he's not allowed only makes him more curious and likely to do exactly what I don't want him to.
You might say just to keep a better eye on him and never leave him alone in the front room. But for reasons unknown to me, people only seem to knock when I'm changing diapers and can't beat little buddy to the door. And he is so quick that the door is open before I'm halfway done saying, "J don't open that door!" Lucky for us, it's usually the mailman or someone from church. But one can never be too careful.