The earth hath bubbles, as the water has, / And these are of them.
Act I, Scene 3 · Banquo
Context
Banquo reacts to the witches' sudden disappearance by comparing them to bubbles that briefly appear on the surface of earth or water before popping.
Analysis
The simile reduces the witches from supernatural beings to natural phenomena—bubbles that rise and vanish—which is Banquo's attempt to rationalize what he's just seen into something explainable. By calling them "bubbles," he classifies them as insubstantial and temporary, draining them of the permanence and authority they've tried to claim. This is Banquo performing skepticism, trying to reassert a materialist worldview that the encounter has just shattered.
Essay Tip
Support a thesis that Banquo and Macbeth respond to the supernatural in opposite ways—Banquo immediately tries to diminish and explain it, while Macbeth ("Would they had stay'd!") wants to preserve and expand it, revealing the psychological difference that will determine their fates.