What has ears but can't hear?

A woman and a baby were in the doctor's examining room, waiting for the Doctor to come in for the baby's first exam. The Doctor arrived, examined the baby, checked his weight, and seeming a little concerned, asked if the baby was breast-fed or bottle-fed.
"Breast-fed," she replied.
"Strip down to your waist," the Doctor said. She did. He pinched her nipples, then pressed, kneaded, and rubbed both breasts for awhile in a detailed examination. Motioning her to get dressed, he said, "No wonder this baby is underweight, you don't have any milk."
"I know," she said, "I'm his Grandma, but I'm glad I came."