At the age of 60, 80 or maybe even 100, the grandmas of Korogocho slum are learning the basics of martial arts for survival in one of Kenya's most dangerous shanty towns.
Only a dozen or so kilometres (eight miles) from the heart of the capital Nairobi, Korogocho's residents -- all 155,000 of them -- are crammed into an area measuring just 1.5 square kilometres.
Most of the young people here survive by scavenging what they can from the giant rubbish heap at neighbouring Dandora.
Even venturing into one of the main streets in broad daylight to queue at a water point is a calculated risk.
So, in this sun-baked universe of mud bricks and rusting corrugated iron, devoid of vegetation, the mat of the "Streams of Hope and Peace" association looks like a haven of tranquility and optimism.
A plastic cover gives some shelter from the sun and the corrugated iron has been painted in bright colours.
Inside, about 20 elderly women, barefoot and clad in shapeless dresses and headscarves, are sitting in a circle on a mat encouraging one of their number who is hitting a punchbag yelling Noooooooooooo!
The blows lack force, but according to the women's teacher Sheila Kariuki, that's not the crucial thing.
"You don't need to hit hard to be accurate," says Kariuki, a youngster by comparison to her charges at 29. "Accuracy is the key to the technique."
She demonstrates the vulnerable points on a young man acting as guinea pig for the class: the nose, chin, collarbone, and of course the genitals.
This self-defence group began in 2007 as way of fighting back at the young bandits of Korogocho who took to raping women three or four times their age.
"Every time the boys make a robbery, they will ask the grandmas to sleep with them," explains Mary Wangui, 73, who was one of the oldest pupils and is now a teacher.
"Young men believe that young girls are all infected with Aids, so they'd rather go to the old ones because they know we don't have any more partners."
The undisputed star of this particular group is Gladys Wanjiku, who thinks she must be "about 100" -- an idea that seems surprising when you see her hit the punchbags with measured, confident blows.
If a man with evil intent comes close, "I will hit him," she says, smiling confidently.
Fortunately, she has been spared attack -- "I pray so much for them not to come," she explains -- but "if a young man wants to rape me, now I know where to hit."
She likes the practice. "I feel so much better, and my body feels so light after the training."
Kariuki, who holds her classes on a voluntary basis, was herself trained by an American woman the self-defence techniques developped by US feminists from the 1970s.
She in turn teaches her elderly students "a mixture of karate, kung-fu and taekwondo."
The group, which has no public money or donations, meets around once a week in a room provided by a charity. Another class is for younger women aged up to 30.
Kariuki harbours no illusions about the chances of Kenyan police providing protection, and so also trains the women how to negotiate and how to overcome their fear.
"I teach them to yell, which is the opposite of screaming," she continues. "When you yell, you are in control, relaxed, telling the world that you don't like what these young boys are doing to you, and telling them to stop."
"The police help but their number is very few," she adds.
"Our programme does make a difference. We have testimonies of old women now able to defend themselves using verbal or physical techniques."
All the women in this class live alone, usually in a rudimentary one-room brick shelter protected, at best, by a little lock. There's no running water, and electricity is patchy.
If anyone tries to harass her, says 70-year-old Joyce Wanjiru, "I will use the negotiation scheme and afterwards I will yell to attract attention."