Why are magnets represented by u?

I remember there seems to be a book that once there was a man riding a horse for a walk. Suddenly, the horse stepped on a big iron block and could not move. When he came down to examine it carefully, he found that the horseshoe was tightly connected with the iron plate. In ancient times, horses' feet were tied with U-shaped iron, so that horses could run faster and longer. ) That man is so depressed that Ba Dou can't move. When he cut off the horse's leg, he found it was. I see. Of course, the facts are groundless. Now we don't have to discuss these at last, just as a joke before tea and after tea.