Ekalavya walked out of the forest and asked Droṇa to teach him archery, and Droṇa said no.
The reason was his birth. He was a forest people's prince, and Droṇa taught kings' sons, and that was the whole of it.
So he went back into the forest, and made a statue of Droṇa out of earth, and put it up, and stood in front of it every day, and taught himself.
For years. Nobody was watching and nobody was correcting him.
And he became extraordinary. Not good. The text says he passed everybody.
The princes were out hunting when a dog got ahead of them and started barking at somebody in the trees.
And seven arrows came, so fast the dog never stopped barking, and filled its mouth without cutting it anywhere. The dog trotted back to the princes exactly as it had left, unable to make a sound.
Arjuna looked at that dog for a long time.
They found him. He was delighted to meet them. He told them, proudly, that he was a pupil of Droṇa's.
And Arjuna went to Droṇa and reminded him of something he had once said — that no pupil of his would ever be greater than Arjuna.
So Droṇa walked into the forest, and found the boy who had built a statue of him, and was greeted like a god. And he asked for his teacher's fee.
The right thumb. The one you draw a bowstring with.
And Ekalavya cut it off and gave it to him, cheerfully, without one second of hesitation, because he had been asked by his teacher.
He went on shooting afterwards, with his fingers, for the rest of his life. He was still good. He was never that fast again, and everybody involved knew that was the point.
He had never been taught a thing by that man. He was the only one in the story who behaved as though he had.