Steven Pinker

An everyday example is driving on the left.

or on the right.

It doesn't matter, as long as everyone agrees to drive on the same side.

As in the joke about the woman who calls her husband during his morning commute and says, Be careful, honey.

The radio says that there's a maniac driving on the wrong side of the freeway.

He says, One maniac?

There are hundreds of them.

Another everyday example is money.

I accept a piece of paper in exchange for an old chair because I know that my grocer will accept it in exchange for some groceries because he knows his supplier will accept it, and so on.

Now, not all the examples are this obvious.

Why are autocrats terrified of public protests?

Well, the basic reason was explained by Gandhi in the eponymous movie, when he said, 100,000 Englishmen cannot control 350 million Indians if the Indians refuse to cooperate.

The problem is that the Indians can't refuse to cooperate if each one fears that no one will join him.

In a public demonstration, each protestor can see the other protestors see the other protestors, and this common knowledge allows them to coordinate their resistance, whether by literally storming the palace or by bringing the apparatus of the state to a halt through boycotts and work stoppages.

This is the basis for a joke from the old Soviet Union, in which a man in Red Square is handing out leaflets to passersby.

Soon enough, the KGB arrest him, only to discover that the leaflets are blank sheets of paper.

They say, what is the meaning of this?

And the man says, what's there to say?

It's so obvious.

The point of the joke is the man was generating subversive common knowledge.