How does the witch take her revenge on the sailor's wife in Act I Sc.3 of "Macbeth?"

Witches (symbolic of women of low status, like gypsies) were "have-nots" in Elizabethan England, the bottom of the Great Chain of Being.  Therefore, they were given little food and had to beg often.  The sailor's wife is a "have," a woman of slightly higher status who hoards her food.

So, the first witch plans to punish the wife through the husband.  Sound familiar?  Are these references to Lady Macbeth and Macbeth?

Witches were thought to be able to kill animals (swine), but not humans.  As proof of her revenge, she shows the thumb to the other witch.  As you know, hand imagery is very important in the play; Macbeth's wife will continually wash the imaginary blood from her hands in Act V.

All in all, the story is a setup for things to come: petty, undeserved revenge is unnatural.  And, don't ever hoard chestnuts from a little old lady.