When shall we three meet again
In thunder, lightning, or in rain?
The witches' first lines differentiate their speech from Shakespearean convention in using a four beat (tetrameter) verse, instead of the standard iambic pentameter. This shorter rhythm gives their lines a sing-song spell-like pattern which, as a prologue to the action, acts like an enchantment cast over the whole play. The second line is also a good example of Shakespeare's tendency to incorporate stage directions in dialogue, so that what characters say often gives us clues to setting or action.