At one time the Blessed One was dwelling at Vesāli in the peaked hall. When Ānanda returned from almsround in Vesāli, he approached Blessed One after the meal and told him that he had seen the Licchavī youths practising archery. They were able to shoot arrows accurately through a keyhole.
The Buddha asked Ānanda, which was more difficult, to shoot an arrow through a keyhole or hit the end of a hair, split seven times.
Ānanda agreed that the latter would be much more difficult to do.
The Buddha explained that to penetrate the four noble truths was more difficult than hitting the tip of a hair divided seven times.
Therefore, Ānanda, an effort should be made to understand the four noble truths. S.v.453.