PSA: Headscratchers is a place to try and find In-Universe explanations. Try to avoid natter, going too off-topic and/or first person language. If a bullet has something you feel is incorrect, just fix it.
How did George and Harold change "Sewage Plant Field Trips are Today" into "Come See My Hairy Armpits"? That's only enough letters for "oe see airy arpits".
I guess they must've brought their own letters to fill in the missing ones.
Maybe the letters for the sign are stored nearby so they just grabbed the extra ones they needed from there.
In one of the books they repurposed some of the letters by rotating or stacking them; they could have done the same here (flipped the W around, for instance).
Why did Poopypants want to become a science teacher, rather than any other occupation more relevant to his world domination scheme?
Considering that the public school system is portrayed as a source of misery and oppression; perhaps the Professor thought a school environment would allow him to work in a distraction-free setting. That, or he just needed a steady paycheck to fund his experiments and took the first vaguely science-related job he could find on short notice.
He said something about it providing him with resources. Also since his evil plan revolved around taking laughter away from children specifically, he had a convenient source of test subjects right there.
While George and Harold were separated after the carnival and were unable to change Mr. Krupp into Captain Underpants because it was raining, why didn't they snap their fingers inside the school?
The books indicate that Krupp has to hear the snapping for it to work.
Perhaps other teachers took them inside before Mr. Krupp entered.
They just finally realized after the disaster of a carnival that, as much as they hated to be separated, it was better than keeping Krupp as Captain Underpants and causing even more damage.
Krupp still would've been wet even after they managed to get inside.
Just WHAT in the WORLD made the school leftovers so toxic and/or outright radioactive in the first place? And considering its what gave Captain Underpants his superpowers, powered the enormous Turbo Toilet 2000 and gave life to the man-eating Talking Toilets, how has it not had any other effects on the students who buy lunch?
There were enough leftovers to fill that huge toilet bowl. It seems very few of the students are that stupid.
How come both Melvin and Poopypants wear full-body hazmat suits to protect themselves when they interact with the leftovers, but Edith, who comes in contact with it far more often and of all people knows just how toxic it is, does not wear any further protective gear other than her rubber gloves?
Maybe the lunch by itself isn't completely toxic to humans, just close enough that the Professor could easily turn it into the fuel he needed.
Edith spends a lot of time around it; she probably built up a resistance over the years.
Why didn't Captain Underpants getting dunked in the Toilet Mech's water revert him to Principal Krupp?
It wasn't water, it was liquidized radioactive cafeteria leftovers.
Where did the mind control ring come from? For that matter, was the story George and Harold told about where the ring came from true, exaggerated for dramatic effect, or completely made up?
George got the thing out of a regular cereal box, and was just making up that story to scare Mr. Krupp.
He wasn't necessarily making it up, just highly exaggerating the origins of the ring.
How come George and Harold never told Captain Underpants that he couldn't get wet? They could have told him that water was his kryptonite or something similar. Maybe C.U. would have had to revert back to Krupp sooner or later, but it would have saved the boys some trouble.
Captain Underpants is really stupid and completely lacks common sense, so he probably would've screwed up and gotten himself wet anyway. Maybe George and Harold are Genre Savvy enough to realize that warning him would be pointless.
Okay, so Principal Krupp separates the boys into different classes. Doesn't mean George and Harold can't still see each other at school, right? Heck, the two live right next door to each other. They could easily still hang out after school, or on the weekends, or even during the summer. So it's kind of a moot point, isn't it? Or is that the point?
Yes, that's the point; they're little kids with short attention spans. They call Monday an eternity away from Saturday, so not seeing each other constantly would do serious damage to their relationship.
Regarding Krupp separting George and Harold: so yes, our protagonists realize at tht end that actually, not being in the same class isn't as bad as they thought. But you'd think that Mr. Krupp, as cruel as he's shown to be, would try a little harder. In the books, he blackmailed them into not only behaving, but doing all his cleaning and filing and stuff.
Movie Mr. Krupp isn't an all-around terrible person the way that Book Krupp was (the book Krupp would never have complimented the boys' comics like he did at the end, for example.) He takes way too much relish in the misery he's causing because he himself is unhappy, but he is technically doing his duties as a principal in separating two problem students.
While it's sweet that Mr. Fyde and Edith received the TAMEST of the pranks from George and Harold, what the HECK did they do to get pranked in the first place, considering they're the nicest of the staff?
Book George and Harold's personalities answer it a bit better: They're a lot less concerned about potential collatoral damage from their pranks and whether the person deserved it. They might have Took a Level in Kindness in the movie, but seemingly some things never change.
When the students, George and Harold lose their sense of humour, why do they have glowing eyes?
When lots of light shines into the eye, most of the light will reflect of the retina, causing the colour of the Tapetum Lucidum (which depends between person-to-person) to shine, making it look like the pupil is glowing. This is known as Eyeshine.
