the guru read this and thought, “if i predict that the tortoise will return the lettuce, he will keep it. but if i predict that he will not return the lettuce, he will be forced into a paradox. for if he returns the lettuce, then he has violated his terms because i did not predict correctly. but if he keeps the lettuce, then he has violated his terms again, because i did. therefore…”
the tortoise used the time the guru spent pondering to escape.