Hiroko walks down the street with her eyes fixed firmly on the pavement. What does her friend's town have to offer in the way of manhole covers? There is one with a picture of a fish, one showing Mount FujiAn active volcano in Japan. It is considered a sacred mountain. , one with flowers, and - yes! - even one with a Pokemon figure. She must put it on Instagram!

Hiroko is one of the thousands of people in Japan known as "manholers". They travel far and wide across the country to photograph manhole covers. Sometimes they take rubbings of them with paper and charcoal.

They collect replica covers and themed items. These range from keyrings to trading cards, which can fetch high prices at auction. They even meet for "manhole nights" to discuss their hobby, and go to exhibitions and lectures about it.

One leading manholer, Hidetoshi Ishii, has cycled to over 1,700 towns and cities to indulge his passion. He has written a book called Manholes of Japan: What Manholes Have Taught Me.

The idea for ornateHighly decorated. designs emerged in the 1980s, when Japan's sewer system needed updating. A clever civil servantCivil servants help the government develop and carry out policies. They are non-political.  saw the covers as a way of winning support for the expensive project.

Today 95% of Japan's municipalitiesA town or district that has a local government. have their own special designs. They feature local landmarks, traditions and tourist attractions. The Tama district of Tokyo has Hello Kitty covers.  

"Little satisfies or showcases the human capacity for ambition, vision, love and connoisseurshipExpertise in a particular subject, such as a type of art. as much as a good collection," writes Leo Lewis in the Financial Times. Among the world's great collectors have been Nancy Hoffman (umbrella covers), Myk Briggs (ear trumpets) and Nick Vermeulen (airline sick bags).

The documentary-maker Jonathan Gili collected everything from pencil sharpeners to sardine tins. When he died in 2006, his hoard was valued at £100,000.

Yes: The value of an object is dictated purely by the demand for it. In the 17th Century, people in the Netherlands paid extraordinary amounts of money for tulip bulbs in an episode known as "tulip fever".

No: You cannot put a price on the things that matter most in life - love and friendship. Even a banknote is valueless in itself: it is a simply a piece of paper whose worth we choose to believe in.

Or... The Irish writer Oscar Wilde was right to define a cynic in his play Lady Windermere's Fan as "A man who knows the price of everything, and the value of nothing."