Karl Barth, Rudolf Bultmann and Paul Tillich are taking a break together, fishing on Lake Geneva. They are having a lovely time, smoking their pipes, chatting idly.
It's hot and they are getting thirsty. So Karl Barth gets up, steps out of the boat, and walks across the water to the shore, gets some beers and returns.
It's quite hot so the beer doesn't last long. Barth tells Tillich: "your turn, Paul". Tillich gets up, steps outside the boat, walks across the water, and fetches some beer.
It is getting really hot now, and the beer is finished once again. Bultmann is beginning to sweat particularly profusely... and finally Barth asks him too: "Come on, Rudolf, your turn now." With a slight tremor in his knees, Bultmann gets up, steps out of the boat, and sinks like a stone. Fortunately he is a good swimmer; he drags himself back into the boat and sulks at the far end.
Tillich turns to Barth and says: "Do you think we should have told him where the stepping stones are?"
Barth looks at him in astonishment and replies: "What stones?"