Daniel Kish has been blind since he was a baby but that hasn't stopped him living an incredibly active life that includes hiking and mountain-biking. To do this, he has perfected a form of human echolocation, using reflected sound waves to build a mental picture of his surroundings.
When Daniel Kish clicks his tongue, the world answers back.
Cars, trees, doorways, bollards on the pavement… all are identified and mapped in his brain using information gleaned from a series of sharp little taps of his tongue against the roof of his mouth, two or three times a second.
From an early age, the Californian developed a sonar technique which allowed him to navigate using echoes from repeated tongue-clicks. The skill has led to him being dubbed a "real-life Batman" - a description he welcomes.
"It is the same process bats use," he says. "You send out a sound or a call and sound waves are physical waves - they bounce back from physical surfaces.
"So if a person is clicking and they're listening to surfaces around them they do get an instantaneous sense of the positioning of these surfaces."
The echoes from his clicks inform Kish about an object's distance, size, texture and density. It's enough for him to differentiate between, say, a metal fence and a wooden fence.
"It's not that I can really tell metal from wood, but I can tell the difference between the arrangement of structures," he says.
"For example, a wooden fence is likely to have thicker structures than a metal fence and when the area is very quiet, wood tends to reflect a warmer, duller sound than metal."
But, he adds, conditions really have to be right to discern this reliably.
Echolocation has allowed Kish to pursue outdoor hobbies such as hiking, despite being totally blind. Kish also says echolocation allows him to engage aesthetically with the world.
"The sense of imagery is very rich for an experienced user. One can get a sense of beauty or starkness or whatever - from sound as well as echo," he says.
"Even architecture has some distinction. One can click at a building, for example, and hear whether or not the building is ornamented or featureless."
Kish now devotes almost all his time to training other blind people in his technique, which he calls FlashSonar. More than 500 students in at least 25 countries have taken the course which is run by not-for-profit organisation, World Access for the Blind.
On one level, there is nothing revolutionary in human echolocation. Emma Tracey, who writes for the BBC's Ouch! disability blog and has been blind since birth, says all blind people use sound as they move around in their daily lives.
This can vary between "passive echolocation", in which incidental echoes are used to help navigate, and "active echolocation", in which the subject emits a noise in order to produce echoes - whether it be a click of a tongue or a tap of a cane.
"You find yourself using your footsteps a bit loudly sometimes to just get your bearings," says Tracey. "Sometimes you click your fingers, almost without thinking."
She says that the echoes created by the sounds she makes vary depending on whether she is in a wide open space or around dense objects, and this helps to inform her movements.
But, she says, using sound to navigate has its limitations. "If it's snowing it's very difficult to get around, very difficult," she says.
Fiona Sandford, who runs Glasgow-based blind support charity Visibility, invited Kish over to train her outreach staff in FlashSonar several years ago.
"Many people who are blind do use a form of echolocation," she Sandford. "And what Daniel Kish does, he takes that ability and hones it."
She likens the impact of his training to a piano student progressing from the ability to play a simple tune to performing a concerto.
And yet it seems that relatively few blind people use active echolocation to the extent advocated by Kish and a few others around the world.
"In many instances it's discouraged," says Kish. "I personally have worked with students who've come from schools for the blind, for whom clicking was actively discouraged.
"I believe it's discouraged because it's seen as a 'blindism' - if you're clicking then you're drawing undue or negative attention to yourself."
Fiona Sandford admits that the clicking noise is a barrier for some of her more self-conscious clients, particularly the adults.
"What we've found is the people that are most receptive to echolocation are young people."
Much of Daniel Kish's work is focused on training children - some as young as toddlers - to gain confidence and independence by using a long cane together with echolocation.
The Royal London Society for Blind People's Dr Tom Pey believes that blind people should be introduced to the technique, but agrees that some people may resist it.
"In a world that is dominated by conservative forces of health and safety you are likely to encounter resistance," he says.
"Some organisations for blind people look at the solution in terms of the service they can provide, as opposed to the service that the blind person requires."
The Royal London Society for Blind People school does offer training in echolocation, but he doesn't think that it is for everyone.
"You have to be able to listen in a certain way in order to interpret the message you are getting back. Not everyone can do that. And clicking may not be for you.
"Not all of us will be able to do what Daniel does but he's shown us that it is possible. I really tip my hat to him - long may he continue to be an inspiration for blind people."