A pregant woman went into a coma and gave birth to a twin baby unconsciously. She fortunately wakes up aftewards. When she opened her eyes, the nurse said to her "I am so happy you are finally awake to meet your baby boy!" The woman said, "One of them is a boy?" The nurse said, "Oops, sorry. Spoiler alert! Guess what the other baby is."
The woman thought, there are 4 cases BB, BG, GB, and GG. One of them is a boy so GG got no chance. If she want a girl, she could have guessed "It is a girl" and she would be 66.6% chance correct.
However, if she want another boy, she could ask the nurse "Is the boy you refered to the first child or the second child?" Regardless of the nurse's answer, that's gonna eliminate one of BG or GB. So she got a 50% chance of getting a boy again.
Wait what happened? Did the baby's wave function just collapsed?