It was a simple act. In 1955 department store seamstress Rosa Parks, on her way home from a long day at work refused to give up her seat in the "coloured section" to a standing white passenger.
It was an act that would light a flame that would burn from Montgomery, Alabama, all across the United States and then across the globe.
Parks's simple act would change the world and open the way, not least, to a black president in the White House.
If you don't believe me ask Barack Obama. He will tell you the debt he and all black people owe to Rosa Parks. He has been leading the celebrations and the tributes to her all this month - 100 years after her birth.
So who was this remarkable woman and how did she come to change the world we all live in?
Parks was born Rosa Louise McCauley in Tuskegee, Alabama, on February 4 1913 to Leona, a teacher, and James McCauley, a carpenter.
When her parents separated she moved with her mother to Pine Level, just outside the state capital of Montgomery. There she grew up on a farm with her grandparents, mother and younger brother Sylvester.
In the southern states black people lived under racist Jim Crow laws, segregation was imposed in public facilities and retail stores, including public transport. Electoral laws effectively disfranchised black voters.
Parks recalled her school days in Pine Level. School buses took white students to their new school and black students had to walk to theirs.
When the Ku Klux Klan marched down the street in front of their house, Parks recalls her grandfather guarding the front door with a shotgun.
Her Montgomery Industrial School, founded and staffed by white northerners for black children, was burned down twice.
In 1932 Rosa married Raymond Parks, a barber from Montgomery.
Raymond was a member of the National Association for the Advancement of Coloured People (NAACP), which at the time was collecting money to support the defence of the "Scottsboro Boys," a group of black men falsely accused of raping two white women.
At her husband's urging, Parks finished her high school studies. Despite the Jim Crow laws she succeeded in registering to vote on her third try.
Parks became active in the civil rights movement, joined the Montgomery chapter of the NAACP, and was elected secretary. She continued as secretary until 1957.
Although never a member of the Communist Party, she and her husband did attend communist meetings. Some were about the Scottsboro case in which several black men had been falsely accused of rape. It was a campaign that had been brought to prominence by the Communist Party.
Parks worked as a housekeeper and seamstress for Clifford and Virginia Durr, a white couple. Politically liberal, the Durrs became her friends.
They encouraged - and eventually helped sponsor - Parks in the summer of 1955 to attend the Highlander Folk School, a left-wing education centre for activism in workers' rights and racial equality in Tennessee.
Close to the Communist Party, the Highlander Folk's School was the place where the old slave ballad We Shall Overcome was turned into the anthem of the civil rights movement and so many other campaigns.
In August 1955, black teenager Emmett Till was beaten and shot after reportedly flirting with a young white woman while visiting relatives in Mississippi.
In November 1955 Parks attended a mass meeting in Montgomery that addressed this notorious case. Discussions at that meeting concerned actions black people could take to work for their rights.
Later that year Parks took her momentous action.
As her usual bus travelled along its route, all of the white-only seats filled up. The bus reached the third stop in front of the Empire Theatre, and several white passengers boarded.
The driver moved the "coloured" section sign back behind Parks and demanded that four black people including Parks give up their seats in the middle section so that the white passengers could sit.
Parks had had enough. She said No and was promptly arrested.
Three days later, on Sunday December 4 1955, plans for the Montgomery bus boycott in her support had been announced. The black people of Montgomery had had enough too.
At a huge rally that night they agreed to continue the boycott until they were treated with a level of courtesy, until black drivers were hired and until seating in the middle of the bus was handled on a first-come basis.
The next day Parks was tried. The trial lasted just 30 minutes. Found guilty she was fined $10 with $4 costs. Parks appealed and formally challenged the legality of racial segregation.
Parks became an icon of the civil rights movement but she also suffered hardships. She was sacked from her job. Her husband quit his job after his boss forbade him to talk about his wife or the legal case.
It didn't stop Parks travelling and speaking extensively about the issues.
Later that year, at the urging of her brother and sister-in-law Rosa, Raymond and her mother moved north to Detroit. Parks worked as a seamstress until 1965.
In 1965 John Conyers, an African-American politician hired her as a secretary and receptionist for his congressional office. She held this position until she retired in 1988.
Her husband Raymond died in August 1977.
In 1987 she founded the Rosa and Raymond Parks Institute for Self-Development. It runs Pathways to Freedom bus tours which take young people to important civil rights and Underground Railroad sites throughout the country.
When in 1994 the Ku Klux Klan applied to sponsor a portion of United States Interstate 55 at St Louis, Missouri, and the state found that it could not legally refuse the racists' sponsorship, instead it voted to name the highway section the "Rosa Parks Highway."
Rosa Parks died of natural causes at the age of 92 in October 2005. She and her husband never had children.
City officials in Montgomery and Detroit announced in a unique tribute that the front seats of all city buses would be reserved with black ribbons in honour of Rosa Parks until her funeral.
Her coffin was taken to Washington DC and transported by a bus similar to the one in which she made her protest, to lie in honour in the rotunda of the US Capitol.
She was the first woman and only the second black person to lie in state in the Capitol.
An estimated 50,000 people viewed the casket there and millions saw it on television.
In later life and after her death Parks received national and international recognition.
She was invited to be part of the group welcoming Nelson Mandela upon his release from prison in South Africa.
Time magazine named her one of the 20 most influential and iconic figures of the 20th century.
President Bill Clinton awarded her the Presidential Medal of Freedom. She was also presented with the Congressional Gold Medal.
Later this month a statue will be unveiled In Washington's Capitol. It will be the first statue of a black woman there.
This month all over the US they are celebrating the centenary of this amazing person.
We too should pay tribute to Rosa Parks - the woman who sat down for freedom.
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