We all know someone with unusual eating habits, or somebody who buys into unconventional diets. Maybe they like to nibble on the end of a pencil when they're scribbling something down, or chew their hair when they're deep in thought. Perhaps they're buying into some internet trend where you only eat celery sticks and poached eggs on weekdays - whatever.

If you can think of it, chances are that someone has already tried to pop it into their mouth and bite down hard. Despite that, I'm pretty sure that most people would baulk at the idea of dining on chalk every day, and certainly wouldn't ever give up their current career to sell the stuff to other chalk-eaters.

Someone who has is Mandisa Mayne, who hails from the city of Birmingham, England. Mandisa has a condition called 'pica' - which causes people to eat non-food substances with no nutritional value, and as a result she finds clay utterly irresistible, and consumes it at an incredible rate.

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Mandisa Mayne's unusual cravings first began in childhood, when she was in school. Mandisa found that she had an overwhelming urge to chew or nibble on pieces of chalk that she'd steal from the teacher's blackboard and nibble it during lessons.

By the time she reached secondary school, she realised that she was addicted. She would steal chalk from the school art department, or and buy her own supplies from artist's shops. Later on in adulthood she became a school teacher herself, where she had unlimited access to all the chalk she could get her hands on.

Commenting on her pica, Mandisa stated: "When I became a teacher, I’d eat clay and chalk in my classroom in my breaks. Before that, working in admin, I remember one time when I was pregnant, when my work building was being refurbished and plaster was hanging off the wall, I couldn’t resist getting a screwdriver and hacking more plaster off, just so I could eat it. It was an uncontrollable craving – I just couldn’t help it."

But soon the chalk just wasn't enough, and Mandisa moved onto clay instead. "I’d got to the point where it really wasn’t doing it for me," she stated. "All the shops I was buying it in were cheap and it didn’t taste like it used to. I tried everywhere to find something which was as good as the old stuff, but I had no luck. I started selling it, as I wasn’t passionate about teaching anymore. I order in up to 40kg a week, keep it all around my house and then distribute it. Now I’m a successful businesswoman, selling it to people just like me. Eating clay makes me feel healthy, it’s just like a detox. I even eat it off a spoon, sometimes, like yoghurt or ice cream."

Yes, Mandisa has now quit her job as an educator, and instead sells edible clays and chalks online. She's doing pretty well for herself too, selling 190 types of clay, chalk and slate pencils to other people with pica, and has already managed to match her 32,000-a-year salary. Mandisa has even been warned by other people that her clay-eating habit could be causing her serious health problems; yet she remains adamant that it's harmless.

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Mandisa stated: "Starting to sell clay was the best decision I’ve ever made. Our house is a little over-run with my supplies and I’m at the Post Office most days, posting it around the globe, but it’s the best job in the world for me."

Okay, so maybe eating chalk does sound a little strange at first. But think about it: is it really that much weirder than chewing chewing gum? That has no nutritional value either. You be you Mandisa.

Mum is so addicted to eating clay that she quit her science teaching job to sell it online