The next morning, before Mr. Wormwood leaves for work, Matilda sneaks into the cloakroom and, using a walking stick, gets her father’s hat off its hook. She carefully squeezes Superglue around the hat’s rim and then puts it back on the rack. When Mr. Wormwood puts the hat on moments later, he doesn’t notice anything—but when he gets to work, he discovers his hat is stuck on his head. And Superglue is strong stuff; it’ll take his scalp off if he pulls too hard. So he tries to look like he’s keeping his hat on deliberately.
Matilda has to be crafty as she goes about embarrassing her father—and ultimately, she’s successful. By successfully gluing his hat to his head, Matilda achieves some degree of power over him. Note that this power isn’t permanent; there’s no indication he knows who’s responsible, and it probably won’t make him change his ways. But at the very least, it will give Matilda—and readers—a chuckle.
When Mr. Wormwood gets home and admits he can’t get his hat off, Mrs. Wormwood tries to yank it off. Mr. Wormwood shrieks and Matilda, nestled in a chair with a book, watches. She innocently asks if her father’s head is swollen. Though Mr. Wormwood suspects his daughter, he knows he has no reason to. Mrs. Wormwood sighs that Mr. Wormwood needs to be careful with the Superglue and not try to glue feathers in his hat while he’s wearing it. Mr. Wormwood shouts at her, incensed at the implication that he meant to glue his hat to his head.
Despite his faults, Mr. Wormwood knows it’d be going too far to blame Matilda for something he believes she couldn’t do. So in a way, Matilda’s power comes from the fact that the adults around her underestimate her. Readers know that she’s capable of such a trick, but she’s never going to have to face up to consequences because nobody in the novel thinks she’s capable.
Matilda helpfully notes that a little boy once got Superglue on his finger and stuck it up his nose. People scolded him for a week to stop picking his nose. Mrs. Wormwood scoffs that picking one’s nose is a nasty habit he shouldn’t have been doing anyway, but Matilda notes that she saw Mrs. Wormwood pick her nose yesterday.
With Mrs. Wormwood’s reaction to this anecdote, Matilda gets another unexpected victory over her parents: embarrassing her mother by pointing out that she, too, picks her nose. She essentially points out that adults have faults too, which is clear to readers after meeting the Wormwoods but doesn’t seem nearly as clear to the Wormwoods themselves.
Mr. Wormwood looks ridiculous as he wears his hat all evening. He can’t shower, and later, as Mrs. Wormwood watches her husband in his striped pajamas and hat, she thinks he looks “stupid.” Mr. Wormwood can’t sleep that night. In the morning, Mrs. Wormwood cuts the hat away. She cuts away a band of her husband’s hair, but in front, patches of leather stick to his forehead. Matilda earnestly tells her father he looks like he has lice; he must try to get it off. The whole thing is very satisfying for Matilda.
Perhaps unwittingly, Matilda is even hurting her parents’ marriage (recall that Mrs. Wormwood seems convinced her husband glued his own hat to his head). The fact that this trick is so successful shows Matilda that she can gain power over her parents. She’s not totally powerless, just because she’s a small child.