One day, Buddha questioned a number of his students,

 "How long can a human life be certain?"

One of the students answered, "For a few days."
Buddha replied, "No, you have not yet understood life."
Buddha then repeated,

"How long can a human life be certain?"

Another student answered, "For a meal."
Again, Buddha replied, "No, you have not understood life either."
Buddha repeated one third time,

"How long can a human life be certain?"

A third student answered, "Only for a breath."
Buddha praised, "Great! You've started to understand life."