"mindswap" Robert Sheckley:
'I understand now,' Marvin said. 'According to the theory, I must find Cathy.'
'Yes,' Valdez said. 'You must find Cathy; as a matter of fact, you must find everyone. The only limitation is the Set-Expansion factor, or S-E.'
'Oh?' Marvin asked.
'Well, naturally, all searches can only be successful during the time, or duration, of S-E. But the duration of S-E is a variable which can last no less than 6.3 microseconds and no more than 1,005.34543 years.'
'How long will S-E last in my particular case?' Marvin asked.
'A lot of us would like to know the answer to that one,' Valdez said, with a hearty chuckle.