Initially the idea for creating an automatic home cleaning device belonged to Arthur Weasley. He had come across a vacuum cleaner and thought the whole idea was fascinating. The entire machine was a bit too large (even for him) to sneak out of the Ministry of Magic, but that didn't mean that he couldn't take it out piece by piece.
Putting it back together proved more of a challenge, so the pieces remained abandoned in a corner, acknowledged only by the occasional longing glance and heavy sigh (the number of these originating from Molly Weasley remains unknown).
Percy Weasley thought it was disgraceful and a sign of his father's strange, unnatural interests. Bill and Charlie Weasley had no opinions at all, both having already moved out of the home. Ron Weasley barely noticed it, being quite a bit more interested in playing Quidditch in the back garden than with cleaning devices, and Ginny Weasley thought it made a good chair.
Fred and George Weasley thought it was a challenge.
The idea behind it was fairly simple, of course. Muggles had impressive imaginations, of course, and it was an idea that they hadn't seen before. Still, the mechanics weren't terribly complicated and they were up to the task. Ginny was annoyed when they took her seat away (she liked it, she'd found a little round pillow that perfectly fit it and in turn fit her quite well when she folded her legs up under her) but otherwise its disappearance went without remark.
That was a mistake.
The first thing to go was, of course, the electrical cord. That was useless. The hoses were inconvenient and bulky, so those went next. A few simple adjustments and inappropriate use of an appropriated knife later, the "vacuum", if it could still be called that, was ready for testing.
They knew that trying it on their mother was out of the equation, they didn't even think of taking that particular risk. Their father would get entirely too much glee, and most likely it would just confuse Ron or Ginny.
Obviously this left Percy.
Cleany had mostly been left to gather dust. This was a bit ironic, but given that both Fred and George preferred their ears unboxed they had given up on using it to annoy the household, and with the elves about to clean it wouldn't have much purpose at Hogwarts.
This was their general opinion until the next term when they had the unfortunate experience of meeting Dolores Umbridge.
It honestly wasn't their goal when they began hunting through old inventions, but the sight of it brought identical smiles to their identical faces. Eyebrows raised, nods exchanged, and adjustments were planned.
Cleany would have to be downsized a bit in order to be packed up, but after a bit of plotting and a lot of stealing they managed to bring it to a reasonable size, conveniently enough at about ankle height. They decided it was also important that new Cleany be quite a bit faster than it the previous model. It had been entirely too easy for Percy to catch and curse the creation before, and it was absolutely vital that Cleany be uncatchable.
It wasn't until after they returned to Hogwarts that they thought of an even better idea.
Assault was a terrible thing, and overtly attacking a teacher would certainly be reason for expulsion, if not arrest. That was why they had to be extremely sneaky about it. They were fortunate that they were so very well liked by the house elves, or it would have been much more difficult to acquire a reasonably sharp knife.
With the borrowed knife, a sticking spell, and one night of intense charm work later, Cleany officially became Stabby.
George could not say precisely what made him think of Stabby at his little sister's wedding, but his proximity to Petunia Dursley probably had a lot to do with it. Her general opinion of the guests, food, decorations and particularly the bride riled him quite a bit, and despite the satisfaction of leaving an entire mischief of Mary's Magical Mice in their car, he still felt a bit restless.
Then he remembered Stabby.
It felt a bit ridiculous to sneak into Hogwarts after so many years (and that pesky expulsion), but he and Fred had never quite got 'round to catching Stabby after setting it loose. Furthermore, he wasn't entirely confident that McGonagall would let him in to look for it given that he was pretty sure it had gone after her ankles at least once.
Nicking the Marauder's Map wasn't terribly difficult because Ginny was absurdly trusting of her big brother, and Harry had terribly uncreative hiding places. Using it alone felt strange and unnatural (as so many things did), but he pressed on for the sake of the greater good. Well, greater 'no good', actually. He'd solemnly sworn on it.
Funnily enough, finding Stabby was absurdly easy. He didn't realize that he had charmed loyalty into it, but he (or maybe Fred?) had done stranger things by accident before. He hadn't been in the school for more than half an hour before it came humming along, knife and all.
Escaping relatively unharmed (much to the amusement of Dumbledore's painting), he brought his project home.
The knife would have to go. As satisfying as it would be, physically damaging the Dursleys was too easy. Any old boy with a wand and a weapon could injure a person, but psychological damage, that was where the real challenge was. So, Stabby temporarily returned to being Cleany. It had been a bit too fragile before - he remembered the day that Percy had cracked the shell with a well placed kick - so that was the first thing to be adjusted. He thought crawling up walls might be a good feature to add. Maybe ceilings, too, just to be thorough.
It was this device that was delivered to Petunia Dursley, with only one charm left to activate - from the moment she touched it, it loved her. Stalky, as it was now called, would follow her to the ends of the Earth. And did. It followed her through the house, happily whirring as it cleaned her carpets. It followed her up the stairs (this was where the ability to climb walls came very much in handy), scrubbing dirt from the paint. It sat above her bed, patiently waiting for her to wake up and making sure that there was no dust on the upper cabinets.
Stalky also turned out to be very good at escaping rubbish bins, sneaking aboard aeroplanes, and cheerfully following cars along the motorway. Stalky, as it happened, was quite fast.
Yes, George thought, psychological was definitely the way to go.