Phantom Baby Cries

Sometimes after all the kids go home I still hear babies crying. I guess it's down to the simple fact that no matter what I'm doing I always have one ear tuned into what the babies need...even when they're not there? Like tonight, alone in my home after everyone left, I was doing the dishes and I could have sworn I heard crying from the room where the kids take their naps. It makes me a little sad I guess.