Here and there on Q&A forums – Yahoo Answers, Whatsthatbook.com, etc. – you’ll see someone asking for the title of that one story they read years ago…the one with the weird candy or something that makes you gain lots of weight, and then the machine that magically takes it away.
I didn’t remember the title either until I Googled (view spoiler)["short story aliens use human fat to power ships" (hide spoiler)], or something like that. And it isn’t even the weirdest story I’ve ever read, but it was just weird enough. And the final lines had such a neat combination of cheerful tone/imagery-- (view spoiler)[Deb and Trinja admiring the ring of blue marks on their wrists, "like delicate tattooed bracelets"  (hide spoiler)] -- and eerie implications.
One day there’s a weird new shop at the mall. It’s just an unsupervised room full of vending machines…that offer FREE ICE CREAM. And this is no ordinary ice cream, ooohhh no. This is the best soft serve you’ve ever had in your WHOLE. LIFE.
Day after day, the shop is still there, still unsupervised, and the Swoodies are still free… and just when you realize how much weight you’ve gained, a new machine appears in the parking lot, claiming to make all those pounds disappear.
A classic case of Too Good To Be True, right? . . . Right?
(view spoiler)[Well, that's the really cool thing about this story. We're given a theory about who's behind the Swoodies and Slimmers, and what their motives are -- but we're never told if it's true. And the theory is from a young boy, the narrator's brother, someone we're socially programmed to take less seriously because of his age. If an adult or older teen had come up with the idea, the other characters might've taken more time to consider it--no matter how random it seems--because adults are expected to have legit reasons for their claims.
And it really is a wild claim:
* Aliens park their invisible ships around Earth, hypnotize the world leaders into acting like nothing's wrong, then drop machines all over the planet.
* The first machines contain magic addictive treats that make people gain lots of weight very quickly.
* The second machines convert each person's extra fat cells into energy, which is then transferred to the alien ships.
* Once the aliens have collected all the energy they need, they'll return to their planet and use it for electricity and stuff.
Oh, and those neat blue marks are the aliens' way of regulating how much fat they collect from each person, because after a certain number of Slimmings, that person's fat won't be as good.
Sure, all the facts could fit into that explanation…but there's no evidence that they do. I guess that's the trouble with invisible ships and hypnotized leaders. Plus, even if all that were true…what exactly is the problem? People get to eat as much as they want for free and then lose all their unwanted weight (the latter for a small fee), and the aliens get an environmentally friendly (besides what goes into making the machines) source of energy for their planet. Win-win, right?
Though…alien-made or not, I guess if people only ate Swoodies they'd have serious malnutrition issues. And what would happen after the aliens got all the energy they needed and left (or, if it was some human organization, what if they suddenly decided to shut the operation down)? Would people go into Swoodie withdrawal? And what if there were people who couldn't get to the Slimmers in time, and now they're stranded with unhealthy levels of Swoodie-weight?
And what would aliens do with Earth money?
Still, all this is just speculation. Just based on what the story actually tells us (through Deb), the only reason the reader has to suspect that something isn't quite right about the Swoodies and Slimmers is Deb's own defensive-but-not-completely-confident tone in support of them. She insists that people couldn't be so gullible as to fall for some alien plot, and "would never sacrifice their freedom and dignity just so they could eat and still be thin." 
And yet, how quickly did she and Trinja get past their first misgivings about going into an empty room, in a quiet part of the mall, because a sign inside promised free ice cream? How quickly did they hit snooze on the warning bells and neon red flags that read, "THIS OVERLY KIND STRANGER WHO WANTS YOU TO GET IN HIS CAR ISN'T REALLY SELLING PUPPIES! RUN!! AWAY!!! NOW!!!!"?
And then, that day when Deb had just finished saying how great it would be if she could eat all she wanted and not gain weight, and TOTALLY ON CUE there was a Slimmer machine in the parking lot, and even with Trinja being all skeptical (for once), and even as Deb was proudly telling us how enlightened people are in 2041 because they've finally admitted there's no way to lose weight overnight…how quickly did she start scrounging for money when the strange lady explained what the Slimmer supposedly did?
Oh, and what was that about humanity being too smart to fall for the voice saying, "Er...ah…pay no attention to the sinister plot behind that delicious ice-cream and weight-loss curtain!"?
Nobody really understands how these slimmers work. The attendants, all just as strange-sounding as the woman in our mall, get so technical in their explanations that none of us can follow the principles they’re talking about, so we don’t much worry about it. […] We’re too happy to want to upset anything by asking questions. 
And besides, those little blue bracelets are so cool.
The story ends on a cheerful note, but if you consider Deb a biased/defensive narrator, you can feel the spooky undertones. She's jumped on the Swoodie wagon, happy to wear their swag and follow their world tour…for as long as it lasts. (hide spoiler)]
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 Cross-roads: Classic Themes in Young Adult Literature. Points of Departure series. Glenview, IL: ScottForesman, 1995. Pg. 272