Shall we dance? You're walking along, through a crowd. Somebody coming the opposite direction has a trajectory that leads directly into you. You look up just before you run into each other, then start to move to one side, just as he moves to the same side. So you start to move the other way, again, just as he does. You smile at each other, and gesture with your eyes to one side, and finally, you move around each other.

It happens thousands of times in every life. The other day, though, I had that experience with a buddhist monk, in full robes, coming out of the subway.