Once upon a time, there was a boy named Clarence that hated Halloween. He hated the bats, he hated the pumpkins, he hated the ghosts and ghouls and goblins.
So when his parents told him that he had to take his little brother out trick or treating, Clarence pretended to be sick. His mother, however, was a neurosurgeon who assured him that it wasn't possible to have a disease that "turned his brain into spaghetti-Os" so very shortly after that episode he was trudging behind his little brother Claude as they went from house to house.
Claude was having a great time. He skipped down the lane, swinging his orange plastic pumpkin candy bucket and doing his best rendition of the Monster Mash. At each house he was complimented on his costume, and at each house he bellow "TRICK OR TREAT!" from beneath his astronaut helmet.
Clarence sulked and kicked piles of leaves in anger.
"It isn't fair," Clarence thought, watching the neighbors lavish attention and candy on Claude. "Just because the stupidhead got dressed up for a stupid holiday and said a stupid phrase doesn't mean that he should get more candy than me."
"BUT IT DOES!" said a ghastly ghost of a goblin that had emerged from the attic of the house where Claude was soliciting candy.
"W-w-what are you?" Clarence asked.
"ONE OF THE CREATURES THAT YOU DETEST SO HARD ON THE HOLIDAY THAT WAS SPECIFICALLY MADE TO CELEBRATE OUR SPOOKINESS," the ghost intoned.
"I didn't think you were real!"
"Oh, but we are. And we don't appreciate your bad attitude. Ghosts provide a valuable service, we fulfill a very specific ecological niche. Without us, a very wretched and terrible thing would take up residence in the attics that we occupy."
"What would they be?"
"SOUR LITTLE BOYS LIKE YOU," the ghost said.
Claude came back and handed Clarence a candy bar.
"This is full size!" Clarence said.
"Yep, got a whole box from that last house," Claude said, gesturing to the big box that he carried under an arm.
The Moral: if you smell mothballs, that means that a ghost is doing his job.