Revolutions can be short and bloody, or slow and peaceful. Each is different, though there are recurring patterns - including some that were on show in Egypt.
Trotsky once remarked that if poverty was the cause of revolutions, there would be revolutions all the time because most people in the world were poor. What is needed to turn a million people's grumbling discontent into a crowd on the streets is a spark to electrify them.
Violent death has been the most common catalyst for radicalising discontent in the revolutions of the last 30 years. Sometimes the spark is grisly, like the mass incineration of hundreds in an Iranian cinema in 1978 blamed on the Shah's secret police.
Sometimes the desperate act of a single suicidally inflammatory protester like vegetable salesman Mohammed Bouazizi in Tunisia, in December 2010, catches the imagination of a country.
Even rumours of brutality, such as the claims the Communist secret police had beaten two students to death in Prague in November, 1989, can fire up a public already deeply disillusioned with the system. Reports that Milosevic had had his predecessor, Ivan Stambolic, "disappeared" in the weeks before the Yugoslav presidential elections in 2000 helped to crystallise Serbian rejection of his regime.Chinese template
Death - though in this case non-violent - also played a role in China in April 1989, when students in Beijing hijacked the officially-sponsored mourning for the former Communist leader, Hu Yaobang, to occupy Tiananmen Square and protest against the Party's corruption and dictatorship.
But although the Chinese crisis set the template for how to stage protests and occupy symbolic city-centre squares, it also was the most obvious failure of "People Power".
Unlike other elderly dictators, Deng Xiaoping showed energy and skill in striking back at the protesters. His regime had made a billion Chinese peasants better off. They were the soldiers sent to shoot down the crowds.
Protests against Suharto's "re-election" in Indonesia in March 1998, culminated in the shooting of four students in May, which set off a round of bigger demonstrations and more violence until more than 1,000 were dead.
Thirty years earlier Suharto could kill hundreds of thousands with impunity. But corruption and the Asian economic crisis had imploded support for his regime. After 32 years in power, his family and their cronies were too rich, while too many former backers were getting poorer - a poverty they shared with ordinary people.
What collapses a regime is when insiders turn against it. So long as police, army and senior officials think they have more to lose by revolution than by defending a regime, then even mass protests can be defied and crushed. Remember Tiananmen Square.
But if insiders and the men with guns begin to question the wisdom of backing a regime - or can be bought off - then it implodes quickly.
Tunisia's Ben Ali decided to flee when his generals told him they would not shoot into the crowds. In Romania, in December, 1989, Ceausescu lived to see the general he relied on to crush the protesters become his chief judge at his trial on Christmas Day.
External pressure plays a role in completing regime-change. In 1989, the refusal of the Soviet leader, Mikhail Gorbachev, to use the Red Army to back East European Communists facing protests in the streets made the local generals realise that force was not an option.
The United States has repeatedly pressed its authoritarian allies to compromise and then, once they have started on that slippery slope, to resign.Sclerosis
Longevity of a regime and especially the old age of a ruler can result in a fatal incapacity to react to events quickly.
Revolutions are 24-hour-a day events - they require stamina and quick thinking from both protesters and dictators. An elderly inflexible but ailing leader contributes to the crisis.
From the cancer-stricken Shah of Iran via the ailing Honecker in East Germany to Indonesia's Suharto, decades in power had encouraged a political sclerosis which made nimble political manoeuvres impossible. As Egypt reminds, revolutions are made by the young.
Graceful exits are rare in revolutions, but the offer of secure retirement can speed up and smooth the change.
In 2003, Georgia's Shevardnadze was denounced by some as a "Ceausescu" but he was let alone in his villa after he resigned. Suharto's generals had ensured he retired to die in peace a decade later - but his son "Tommy" was imprisoned.
Often there is a hunger among people to punish the fallen rulers. Their successors, too, find retribution against the old leader can be a useful distraction from the economic and social problems, which don't disappear with the change of regime.