The teacher was leading the youthful class in mathematics.
"No," she said, "in order to subtract, things have to be in the same denomination. For instance, we couldn't take three pears from four peaches, nor eight horses from 10 cats. Do you understand?"
There was assent from the majority of the pupils but one little boy in the rear raised his hand inquiringly.
"Well, Bobby," asked the teacher, "what is it?"
"Please, teacher," he said, "couldn't you take three quarts of milk from two cows?"
Source: J. Braude's Handbook of Humor for All Occasions, 1958, p. 24