"Punish me," read the waterproofed sign which the woman held. Early-morning passers-by gave her strange looks, sat in a large plastic box in the middle of the high street. Several stopped to stare or take pictures. Most people's attention was drawn by the large red button attached to the front of the box, like one that might trigger an explosion in an action film.

"What does that do?" One asked her, but she didn't reply - that wasn't the point of the experiment.

The woman's name was Lorna. She had worn a blue flowery dress for this event, which showed off her slim figure and was neither too conservative or too showy, too demure or too sexual. Her blonde hair was neatly tucked behind her ears, but for all intents and purposes, she looked like a woman you might see everyday.

She discreetly checked the timer beneath her seat - quarter past eight in the morning. That left only one hour and forty-five minutes before the lock would release and she could leave this box, in whatever state she was in. There was nothing to do but sit and wait until then.

The first person she thought might press the button was a young man on an e-scooter. "What's this then?" He demanded. "Are you some kind of actor?"

She didn't reply - the idea was that people revealed their true nature without any intervention. Was the average person a cynical sadist like the Stanford Prison Experiment might have you believe? Or - as she hypothesised - was goodness and empathy their default setting?

Eventually, the man jumped back on his scooter. The street was getting busier now with commuters on their way to work or students heading to college. The hardest part was keeping her perfectly neutral expression as people gawped at her and asked questions. Her hands were starting to ache from holding the sign. But no one stepped forward; no one pressed that button that promised to humiliate her in some unknown way.

"What have you got to be punished for?" It was a sharply-dressed man who she imagined working in IT or tech. He gave her a sleazy grin - more of a leer really. "Have you been naughty? Well, you don't have to ask twice."

He leant forward and pressed the button. Instantly, a wave of green gunge cascaded from the hidden mechanism at the top of the box. She flinched as it hit her head, her shoulders, her chest, completely smothering her face and hair. She'd specifically requested thick gunge to be as unpleasant as possible. The deluge only lasted a few seconds, but it felt much longer, even though she'd been prepared for it.

The onlookers screamed in surprise, while Lorna carefully cleaned her face and the sign. The man who'd pressed the button almost pissed himself laughing. Well, there was one crack in the theory that all people were good. The fact that there were hidden cameras recording the entire ordeal didn't make it any more fun for Lorna.

More people gathered around now they knew what the box did. A student who looked eighteen or nineteen dithered for a minute, before pressing the button too. He looked transfixed as the next wave of slime enveloped her again, undoing all the hard work she'd done cleaning herself. The used slime was simply recycled into the top of the box, meaning there was a potentially unlimited supply. The way the student watched her suggested this might be his masturbation material for the next few weeks.

The clock struck half eight, and Lorna shivered in the plastic prison.

When planning the scheme, she'd wondered if the messier she got, the less likely people would be to press the button. In fact, it seemed to be the opposite. The social taboo was broken, and more people were lining it up to press it. A boyfriend and girlfriend did it together. One man with a wicked grin did it twice.

She noticed lots of people taking pictures, and the news soon seemed to spread on social media. Lots of young people were pointing her out - "There she is!" - and making a beeline to get their own clips of her humiliation. They would gladly gunge her for some online likes.

It was a welcome relief to see Joel as the clocks hit nine o'clock - finally halfway through. He'd been her tech support for the project, building the gunge tank, setting up the cameras and monitoring it all from a hidden location.

"How's it going?" He grinned, seeing her appearance. Her hair was sopping, her face was perpetually covered in slime and the blue dress was no longer blue.

"Slower than I thought," she admitted.

"Well I've just come to change the tank, and give you a bit of variety," he said, lifting a heavy box to the mechanism over her head. "Let's give it a test, shall we?"

To her horror, he hit the button, and her world was instantly swallowed by thick, clingy brown slime with the consistency of mud. Onlookers recoiled in disgust. Eight, Lorna thought, updating her mental tally.

A guy wearing a tracksuit was next to approach. "How'd you like some more of that love?" He taunted her.

"Please don't," she said, and instantly hated herself. She promised that she wouldn't interfere, and now here she was begging. But that just seemed to egg him on. He hit the button with glee and recorded as a new wave of chunky blue slime descended on her. Great, now I look like a smurf.

To add to her self-inflicted humiliation, a news crew appeared a few minutes later. It must have been a slow newsday. They asked her questions about the stunt, which she didn't answer, and then filmed a short clip to camera.

"No one knows why this mysterious lady in a box has appeared here today," the reporter said - her glamorous appearance making Lorna look all the worse by comparison. "But she's certainly making a splash. Let's see what happens when we press the button."

The cameraman grinned broadly as he caught her next gunging on tape - bright orange this time. That makes ten. "Check the six o'clock news, you'll definitely be on it!" He told her.

'Punish me', Lorna's sign said, although it was barely readable anymore. Well, the public had certainly done that. There were several points where she would have admitted defeat if the box's door hadn't been magnetically sealed. Finally, at ten o'clock they pinged open. Lorna had been gunged a total of fifteen times, and now resembled a walking blob of colourful plasticine with all the layers of rainbow slime.

Joel appeared holding a dressing gown and a towel as she gingerly stepped out, brushing the excess slime off her. Several people applauded - possibly sarcastically. She and Joel concluded that there was only one way to get her clean without leaving a trail of slime through the city centre, and poured several buckets of water over her.

"Do you want to know the good or the bad news first?" He asked.

"The good," she said, teeth chattering.

"Well I think you've definitely made a powerful statement about the morality of humans and their capacity to enjoy the humiliation of strangers. And the footage looks great."

"And the bad?"

"The microphones got damaged by the first gunge, so basically none of the footage we have is usable. You're going to have to do it again."

As she stormed off, he called after her: "How about pies this time?"