kudos to whoever came up with the original joke
ex and a constant were walking down the street. Suddenly, the constant notices a differential operator walking along the other side of the street. "Oh, no!" exclaims the constant. "I've got to run away! You've got to hide me! There's a differential operator... he could reduce me to nothing!" "Hmmmph," came the haughty reply. "I'm ex. He can't do anything to me." So ex walked across the street and introduced himself. "Hi. How are you doing? I am ex," he bragged. "Pleased to meet you," replied the differential operator. "I'm d/dy."