The Bodhisatta was once a tree fairy, and he witnessed the following. Two fish, one from the Jumma River and the other from the Ganges, met at the confluence, and they complimented each other on their beauty. This turned into a quarrel about who was the most beautiful. Unable to settle the matter, they saw a turtle and asked him to decide. The turtle answered that both fish were beautiful, but he himself was more beautiful than either. This non-answer upset the fish.
In the Lifetime of the Buddha
The fish were earlier births of two young disciples of the Buddha. They regularly bragged about their looks, and one time they debated which of them was the most handsome. They asked an elder (the turtle was an earlier birth of him) to settle the matter, and he said he was more beautiful than either of them. This non-answer upset the young men, and they sulked off.
When the Buddha heard some other disciples discussing how the elder had shamed the vain young disciples, he told them this story so they knew that these two men had also been full of themselves in the past.