Harvey Karp

Is your baby opening his mouth and rooting? (This could indicate hunger.)
Is he yawning, rubbing his eyes, moving his head from side to side, or staring out with droopy eyelids? (This could indicate fatigue.)
Does he seem to be intentionally looking away from you or starting to hiccup? (This could indicate overstimulation.)
Is he making facial grimaces and trying to bear down? (This could indicate intestinal discomfort.)