A teacher asks her class how long a person can go without sleep. A boy answers: "Two days!". A girl says: "5 days!"
Clive however says determently: "3 weeks minus 15 minutes!"
His teacher is surprised by that answer and ask him how he's come by it.
Clive says: "Yesterday, I heard my father say to my mother: 'Lets go to bed 15 minutes earlier, it's been 3 weeks.'"