In the Grimms’ fairy tale Der Froschkönig oder der eiserne Heinrich, a princess loses her golden ball in a well. A frog brings it back after she promises him her companionship. When the frog later knocks at the castle door, she shuts the door on him. Her father, the king, tells her: Was du versprochen hast, das mußt du auch halten; geh nur und mach ihm auf. (What you have promised, you must keep. Go and open the door for him.)