A wooden beam will appear quite solid and unchanged as termites chew away within it. But there comes a time when one step can bring down the whole rotten house.
In May of 1968 the whole structure of capitalism in France was in jeopardy.
When students and workers erupted into action in the first week of May, few had seen it coming. It started with student protests on a variety of concerns including overcrowded classes. Within days there were barricades in the streets and an insurrectionary mood throughout the country.
So what disguised France's revolutionary potential?
The long boom after World War Two meant bosses could concede some important but limited enhancements to workers, and still extract fabulous profits.
This allowed trade unions and workers' parties to grow in confidence and size. But it also caused many workers' leaders to conclude capitalism could sustain gradual improvements in workers' lives, perhaps forever.
And some so-called Marxists believed the boom had pacified the working class in the advanced capitalist countries. That it was only students, or the peasants and workers of the neocolonial world, who would fight.
The supporters of Militant, forerunner of the Socialist Party, were almost alone in rejecting these skin-deep analyses. Mass movements of workers are coming, they said.
No period of economic or political stability can last in a society built on blind competition and the exploitation of the majority by the super-rich.
Like today, the working class and youth everywhere had a litany of grievances - social as well as economic. And, like today, society was increasingly polarised.
In 1958, France's ruling class had brought back strongman General Charles de Gaulle as president, unelected at first, to resolve a major political crisis. His repressive regime helped prepare another one.
On Thursday 2 May, a group of activists faced trial in university courts.
There had been student demonstrations and occupations since 1967, in particular at the University of Paris Nanterre. The soulless Nanterre campus was established on the capital's outskirts in 1964 as an overflow for the famous Sorbonne.
Students were still overwhelmingly from middle class or ruling class families. But the university population had risen from 123,000 in 1946 to 500,000, without staffing and lecture halls to match. Gender segregation rules in accommodation compounded frustrations.
The university administration feared students would meet the trial with mass protests. The dean of Nanterre ordered his campus shut.
So on Friday 3 May, students instead descended on the Sorbonne itself, in Paris's Latin Quarter on the Left Bank of the River Seine. Terrified by the presence of 300 peaceful protesters, the university's rector asked police to clear the square.
They did. With the help of thugs from the CRS riot cops, they stormed in to make mass arrests. Passers-by disgusted by the heavy-handed intervention replied with cobble stones and bottles.
Police made 596 arrests. The lecturers' union, SNE-sup, called a strike in support of the students.
On Sunday 5 May, as the lecturers' strike spread, the state jailed seven students and banned demonstrations in the capital. Demonstrations went ahead.
Students began to occupy the universities on Monday 6 May, demanding their reopening. School students burdened with endless exams joined the protests.
The Communist Party of France - a Stalinist organisation that retained historic authority among workers - had denounced the students. Demonstrators were unrepresentative "groupuscules" or even CIA provocateurs.
The 6 May demonstration of 60,000 in the Latin Quarter proved otherwise. "We are a groupuscule!" they chanted in defiance.
The CRS charged the crowd, and the ensuing violence hospitalised 739 people. Protesters started to build barricades for self-defence.
The attacks inspired mass sympathy for the movement, even among middle class 'pillars of society'. One poll reported 80% popular support in Paris.
A doctor who had watched the clashes from his window wrote to Le Monde newspaper. "Tomorrow there will be police denunciation of 'foreigners' and the real demonstrators will have smashed the cops and I say this with satisfaction."
What was going on? The student movement in France was not unique. Students in many countries were engaged in mass protests for better conditions and against the Vietnam War.
In Germany the student movement was more militant still. Why didn't that blossom into revolutionary upheaval?
France had unprecedented access to consumer goods and a growing economy. But workers saw inflation, VAT rises and deregulation of rents eroding their wages. Strikes had broken out in many places in the months preceding the great strike of May '68.
And de Gaulle's decade of arbitrary arrests, press censorship, rising unemployment and slum housing had stored up mass resentment. Workers in France were spoiling for a fight.
There were unrelated strikes throughout this week. Paris bus drivers struck against longer hours. Metalworkers at Sud Aviation struck against job cuts. Postal workers struck for more pay.
And by Tuesday 7 May, young workers had joined the students. Some arrived with their pneumatic drills to dig up paving stones for barricades.
Several days of fighting took place as young workers and students battled police for control of the streets. This would reach a high point in the bloody 'Night of the Barricades' on 10-11 May.
A division was already opening in France's frightened ruling class. Would making concessions defuse the movement - or was it better to meet it with more violent repression? Such a split is one of Lenin's four classical preconditions for revolution.
Meanwhile, unable to withstand the titanic pressure rising beneath them, the leaders of the major union federations and the Communist Party were about to call a one-day general strike.
They thought this would safely vent accumulated anger. That they could straightaway return to their comfortable existences without the disagreeable work of confronting capitalist power.
They were wrong.