Note to Romeo and Juliet, 1.3.45: "how a jest shall come about"

Romeo and Juliet,
Act 1, Scene 3, line 45.
how a jest shall come about: how something spoken in jest comes true. The Nurse's husband asked little Juliet if she would fall on her back when she was wiser, and she—not understanding the sexual meaning of the question—answered "Aye." That was the jest; now the jest has "come about," because it's time for Juliet to start thinking about a husband, and falling on her back with him.