The trouble with a tipping point is that no one knows where it is.
Protests against the Syrian president Bashar al Assad are continuing, and the embattled leader is trying both force and dialogue to shut them down. So far, the tipping point has not been reached - and it is Mr al Assad himself who will provoke it.
Syria's president is trying to show that he is listening: he has abolished the Supreme Security Court and lifted the state of emergency, reacting to the demands of the people.
He is also trying dialogue with powers abroad in the face of sanctions and political pressure from Europe and the United States, with Syrian officials reportedly telling the International Atomic Energy Agency that they are willing to co-operate with the United Nations' investigation into the building of a suspected nuclear reactor.
But Mr al Assad's default setting has been force, and the killings go on. Nearly 1,000 people have been killed over the past two months of protests; Syrian forces have used tanks and heavy weapons against civilians populations; they have opened fire on mourners; they have forcefully broken up demonstrations; and still the protests continue across the country.
There are now two likely ways for the protests to succeed in removing Mr al Assad. Both require a tipping point that seems still far away. The first is that Sunni soldiers, who make up the bulk of the army, become convinced that their future lies without Mr al Assad and push the Alawite regime to collapse. Another possibility is that the middle class, who have so far not joined the street protests en masse, change their minds and come out in force. Either would signal the end of Mr al Assad's rule - but neither looks likely today.
What needs to change is an event - an unknown event - that tips those groups away from Mr al Assad. Yet it is only Mr al Assad himself who can tip them over the edge.
In Tunisia, there was no one moment when the protests that followed the death of Mohammed Bouazizi, which had been localised, suddenly spread and the middle class took to the streets. It was a combination of the killing of protesters and a general strike by the Tunisian Bar Association over the treatment by the state of lawyers representing protesters.
In themselves, neither was especially momentous - there had been killings of protesters before and other associations, such as trades unions, had rallied against the state. But somehow, the combination of these events convinced the middle class, the silent majority whose acquiescence kept Zine el Abidine Ben Ali in power, that their future lay without his rule.
What the Tunisian situation showed was that the middle class saw clearly that the regime would attack even them in order to ensure its survival, a point when fear of the regime tipped into anger.
Syria is not at that stage yet. The regime of the Assads has been built on a tiny, militarised elite and the co-opting of the middle class, mainly Sunni Muslims and Christians. The government is still calling the protesters troublemakers and terrorists, banking that most of the middle class will look the other way. There is some evidence this is working.
If a change comes, it is most likely to be sparked by something the regime does. As things stand, a divided, leaderless protest movement can do little to persuade the middle class or Sunni soldiers to join them. Rather, it will be an action by Mr al Assad or Assad loyalists that provides the tipping point.
The trouble is, Mr al Assad doesn't know what it is. He cannot calculate - because no one can calculate - what the move will be that finally tips the balance.
It could be the death of Hamza al Khatheb, a 13-year old arrested a month ago in the southern city of Deraa, whose body was released to his family three days ago, reportedly bearing signs of torture. Hamza could yet become a rallying point for Syrians, who see in his treatment the capricious brutality of the regime.
It could be something entirely random, such as the killing of Neda Agha Sultan in Iran during protests against the 2009 election, a middle-class girl in the wrong place who remains a rallying point for Iranians opposed to the government of Mahmoud Ahmadinejad.
Or it could be an incident that is tragically common, such as the death of a young Egyptian called Khaled Said after his detention by security forces last year. When photos of Said's brutally disfigured face were made public, it galvanised anger against the regime of Hosni Mubarak.
Both Neda Sultan and Khaled Said were young, in their 20s, entirely undeserving of the horrific manner of their deaths. What they represented to their respective publics was the randomness of the regime - a randomness that enough people in the society felt might one day reach them.
And yet Mr al Assad cannot stop. By acting decisively and brutally to ensure his regime's survival, he may precipitate his own downfall. The longer the protests drag on without resolution, the more the army clamps down, the greater the chance that an unexpected miscalculation will push Syria over the edge.
Mr al Assad could yet survive the protests, limping on for months or even years. There is an outside chance he could emerge stronger, but as the Arab Spring gives way to a long, hot summer of protests, the odds are against him.
The rule of Mr al Assad is swaying. Buffeted by the people, his regime is looking for a way to gain a sure footing. But in frantically looking for traction, he may yet slip.
