Two kung-fu masters were carrying on about their
respective skills. "Why, my reflexes are such that you will not
believe," boasted Master Foo and drawing his sword, he sliced at a passing fly, which
promptly dropped dead in two pieces. 
"That's nothing," said Master Koh.
Drawing his sword, he made two deft cuts at another passing fly. 
Master Foo was
highly amused. "What are you talking about?" he sneered. "That fly is still
flying." 
"Ah yes," replied Master Koh, "but now it can never
have children."