Final answer:
When Sir Gawain cuts off the Green Knight's head, the Green Knight's head falls off but reattaches.
Step-by-step explanation:
When Sir Gawain cuts off the Green Knight's head in the medieval romance 'Sir Gawain and the Green Knight,' the Green Knight's head falls off but reattaches to his body. This supernatural event is a test for Sir Gawain's bravery and honesty. It symbolizes the theme of chivalry and the importance of upholding one's promises.