What's known as the Arab Spring was initially set off by the self-immolation of a Tunisian street vendor protesting against harassment by local officials. Authoritarian regimes are typically strong, but they can often be surprisingly brittle too. Having no easy way to deal with discontent other than by repression, they can prove vulnerable if some event or circumstance finally acts as a trigger to persuade an unhappy population to turn on its rulers. And the greater the degree of control exerted by a regime, the greater the responsibility it will be seen to bear when things go wrong.