Final answer:
Macbeth and Banquo initially think the witches' prophecy is a joke because they doubt the witches' credibility and find their prophecies absurd and fantastical. Their skepticism turns to curiosity as the prophecy of Macbeth becoming Thane of Cawdor comes true.
Step-by-step explanation:
In Act 1 of Shakespeare's Macbeth, Macbeth and Banquo initially regard the witches' prophecy with skepticism. Macbeth questions the witches' credibility, noting the current Thane of Cawdor is alive and the idea of being king is beyond belief. Banquo, on the other hand, wonders if they have consumed the insane root that affects one's reason, indicating disbelief.
Their doubt is reflected in Banquo's questioning whether the devil can speak the truth, and Macbeth's astonishment at being dressed in borrowed robes, a metaphor for assuming a title not rightfully his. As rational and honorable soldiers, they find the witches' appearance and prophecies bizarre and otherworldly. However, the mockery they initially show towards the prophecies - thinking them as fantastical and absurd - takes a serious turn as the first prophecy comes true and Macbeth is named Thane of Cawdor.
Learn more about Witches' Prophecy in Macbeth