Stop-knopf

Had some friends over for lunch the other day, one of them bringing an attention-seeking baby. As soon as she was left alone, she was crying. When I went over, stared at her disapprovingly and said shut up, she smiled. Then I inserted the "stop-knopf" (teat) and she was quiet for a while until she lost it. How difficult can it be? It's what I always thought. Babies are not very intelligent.