The Buddha once asked a student, "How long is a human life?" The student replied, "It is so brief it seems but a day long." He then asked another the same question, "What is the true length of a person's life?" She answered thoughtfully, "It is the time taken to eat a single meal". And so the Buddha asked a third student, "How long is life?" "The time in a single breath", was the student's reply. "Exactly so." said the Buddha, "You understand".
from Kindness; A Treasury of Buddhist Wisdom for Children and Parents