Rugrats play "Follow The Leader", with Dil being the leader. When Dil sucks his thumb (mimicking Stu when he sucks his own thumb after hitting his hammer on it), the Rugrats do the same, but fail to stop. When the adults noticed this, they panicked after reading in a Lipschitz book that thumb-sucking leads to poor academic skills and bucked teeth. Angelica uses this advice to get Chuckie to believe that they'll turn into ducks, when they get "duck (thetvdb.com)