My view is that friendship permeates human life and is involved in almost everything we think, feel, and do. For that very reason, there is no behavior that is characteristic of friendship. Two people can engage in the very same behavior - visiting someone in hospital, for example - and yet only one of them might be doing so out of friendship; moreover, friends can be doing absolutely anything together, even quarrel or fight. That means that it is difficult, if not impossible, to recognize a friendship simply on the basis of what people do.