In Hastinapur, Guru Dronacharya taught archery to the Pandava and Kaurava princes. Of all his pupils, Arjun was the most dedicated.
One night Arjun was having his dinner. A gust of wind came and blew out the lamp. Everything went dark, but Arjun's hand kept finding his mouth, out of habit. Arjun thought, "If practice can guide my hand in the dark, then practice can help me aim in the dark too." From that night on, he began to practise archery even in the dark. Hearing the twang of a bowstring at night, Guru Drona came to see. He hugged Arjun and said, "I will make you an archer with no equal in the whole world."
Some time later, Guru Drona decided to test his pupils. He had craftsmen make a make-believe bird and placed it at the top of a tall tree. Then he called all his pupils and said, "When I give the word, you must bring down this bird's head with an arrow. Each of you will get a turn."
Yudhishthir was first. He lifted his bow and took aim. The guru asked, "What do you see? The tree, me, your brothers?" Yudhishthir said, "Yes, Guru ji. I see the tree, I see you, I see my brothers, and the bird too." The guru said, "Step aside."
Then came the turn of Duryodhan, Bhim and the other princes. They all said nearly the same thing: "We see the tree, you, our friends and the bird, everything." The guru asked them all to step aside.
Last of all, Guru Drona smiled and called Arjun. Arjun drew his bow and took aim. The guru asked, "Arjun, do you see the bird, the tree and me?" Arjun said, "Guru ji, I see only the bird. Not the tree, not you."
The guru asked again, "Very well. What does the bird look like?" Arjun said, "I can see only its head, not even the rest of its body."
Guru Drona was delighted. "Shoot!" he said. Arjun's arrow flew straight, and the make-believe bird's head fell to the ground. Guru Drona hugged Arjun close.
Today, many families tell this story as "the bird's eye". Its message is clear: whatever you do, give it your whole attention.
The lesson Whatever you do, give it your full attention.
Source: Mahabharat, Adi Parva, Sambhav Parva, Drona's teaching of the princes (Ganguli's translation, sections 134–135) (retold)