The answer is chewing gum. It's a blight on city streets, expensive for local authorities to deal with, and takes a heavy toll on the environment, according to its critics.
It costs about 56 million pounds ($79 million US) a year in England alone, the UK government reckons, to rid the streets of this form of litter. Only cigarette butts are more prevalent.
But one woman in Britain is performing modern-day alchemy on it - Gumdrop founder Anna Bullus.
After eight years of research, including working with materials scientists, Bullus found a way to compound discarded gum into pink pellets called Gum-tec.
Founder Anna Bullus told the Thomson Reuters Foundation chewing gum is made from a synthetic rubber called polyisobutylene.
She said: "One of the things we use it for is the innards to bicycle wheels. It's an incredibly useful material and it's something that is just ending up on our streets when actually we could be doing something better with it."
For its source material, Gumdrop relies on the public to place chewed gum into its hot-pink bins that sit at 600 locations such as train stations, theme parks and universities - a fivefold increase in bin sites since it started in 2015.
It makes money by charging councils for that service.
It also works with some on innovative fixes - such as on Kensington High Street in west London, a tourist hotspot because of its proximity to Kensington Palace, former home to the late Princess Diana.
There the council handed out 5,000 strikingly pink keyring balls in which people could store used gum. When full, the chewer could mail the orb free of charge to Gumdrop; for every three sent in, Gumdrop would send them another.
Gumdrop also put up hard-to-miss football-sized hot-pink bins - which, naturally enough, were made from recycled gum.
The result was a 90 percent drop in gum litter in the Royal Borough of Kensington and Chelsea, according to Suez, the company contracted by the council to clean gum off its streets.
It's also been successful at the University of Winchester where a number of the bins are placed.
"It has certainly reduced the gum in those high traffic areas and hopefully under the table in lecture rooms as well," the university's environment officer Liz Harris said. "I know our cleaning team and our portering teams have said it's noticeable they're not having to clean up as much, and it's saving us money as well."
Processing of the gum is done by third parties. Gumdrop then sells the transformed material to firms that make all manner of items that more commonly use plastic or rubber, including ski-jacket toggles, flip-flops and reusable coffee cups.
"We're working on at the moment with a UK-based shoe manufacturer to make shoe soles that are hopefully due to come out later this year. We can do all sorts of different colours," said Bullus.
Gumdrop makes its own products too, including wellington boots.
The company's volumes are significant: last year the company, which has just three employees, recycled 25 tonnes of gum.