In The King’s English (1997), Kingsley Amis cites an old joke that illustrates the confusing distinction between shall and will:

A swimmer in difficulties was heard to shout, ‘I will drown and nobody shall save me.’ At an inquest on the unfortunate fellow, English jurors wanted a verdict of suicide, Scottish jurors a verdict of death by misadventure, and MacTavish pressed for a rider or footnote rebuking witnesses for making no effort to rescue the victim.

Under the old rule, I shall indicated a prediction and I will denoted a promise or threat. Confusingly, in the second and third persons these meanings were reversed, so that you/she will indicated simple futurity and you/she shall denoted an intention or command. Still more confusingly, old-fashioned speakers of Scottish English reversed this whole understanding of the matter. So while the English jurors thought the swimmer was saying, “It is my intention to drown, and it is my express desire that nobody try to save me,” the Scots took him to say, “I am going to drown and it seems that nobody is going to save me.”

All this has only grown more confused with the popularity of contractions such as I’ll and you’ll, and Americans have generally dispensed with shall and use will for everything. Of the joke, Amis writes, “Nobody tells that one today.”