The Bodhisatta was once a king’s advisor. When the king died his advisors had to choose a new king, and they planned to pick his young son. But the Bodhisatta did not agree with their decision because the son was extremely lazy. So it was decided to have a mock trial in front of the boy, and they intentionally made the wrong decision. When they asked the son if they had decided correctly, the boy curled his lip. This made the Bodhisatta think he had seen through their ruse and was in fact very wise. The next day they arranged another trial and this time judged it correctly. Again they asked the boy what he thought, and when he made the same lip curl the Bodhisatta knew he was in fact a blind fool. So the Bodhisatta was made king instead.
In the Lifetime of the Buddha
The lazy son was an earlier birth of Laludayi, a disciple of the Buddha who was quite stupid. One time after Sariputta and Moggallana, the Buddha’s two chief disciples, had discussed a question the other disciples praised them. But this foolish disciple thought his knowledge was equal to theirs and curled his lip in scorn.
When the Buddha heard the other disciples discussing his disrespect, he told this story so they knew Laludayi had also shown his foolishness through lip curling in the past.