Karṇa knew the request was coming.
That is what makes this story difficult.
He had been born wearing natural armour and earrings. They were not ordinary possessions added later. The Mahābhārata describes them as part of the protection with which he entered the world, and while he possessed them he was extraordinarily difficult to defeat.
Karṇa was also known for another practice. At midday, after worshipping Sūrya, he would give to brāhmaṇas who came asking. His reputation was that he did not send such a petitioner away empty-handed.
Then Sūrya warned him.
The warning came before Indra arrived. Indra, father of Arjuna, intended to approach in the form of a brāhmaṇa and ask for exactly the things Karṇa should least want to surrender: the armour and earrings born with his body.
Sūrya explained why. If Karṇa kept them, even Arjuna would struggle to defeat him. If he gave them away, that protection would be gone.
So Karṇa was not being asked to make a blind gift. He had advance notice of the request, the petitioner and the consequence.
Sūrya urged him to refuse.
Karṇa answered that the loss of reputation for truth and generosity frightened him more than death. He did not want his vow to mean that he gave only when the gift was cheap.
He even asked Sūrya to permit him to keep the promise when Indra came.
Sūrya then gave different advice. If Karṇa was determined to give away the armour and earrings, he should not give them without asking something in exchange.
Ask Indra for his unfailing dart, Sūrya said — a weapon that would not fail against the enemy for whom it was used.
Karṇa accepted that counsel.
The next day, after his worship, the expected petitioner appeared.
Indra came disguised as a brāhmaṇa and asked for the armour and earrings.
Karṇa did not behave as though he had been fooled. He knew who was standing in front of him and said what the gift would mean: without the armour and earrings, his destruction became possible.
He offered an exchange.
If Indra wanted the protection born with Karṇa's body, then Indra must give him the dart.
Indra acknowledged that Sūrya had revealed the plan and agreed to the exchange, while setting conditions on the weapon.
Karṇa received the dart.
Then he gave Indra the armour and earrings.
The text does not hide the physical cost of that act. The protection was body-born; surrendering it was not like handing over a necklace from a chest.
Nor does the story hide the strategic cost. Indra wants the gift precisely because Arjuna will benefit from Karṇa becoming vulnerable.
That is why the episode cannot honestly be reduced to sharing is good.
Karṇa's generosity is remembered because this is the point where generosity and self-protection collide. Sūrya gives him every reason to draw a boundary. Karṇa hears the reasons and chooses not to draw it there.
At the same time, he does negotiate. He is not careless about the future and he does not pretend the danger is unreal. He asks for a weapon in return.
A child does not have to copy Karṇa's choice in order to understand why the epic remembers it.
The real question is harder: what should happen when a promise that seemed noble in ordinary circumstances reaches a cost you never want anyone you love to pay?
The warning itself comes to Karṇa at night, with Sūrya appearing in the form of a brāhmaṇa during a dream. Karṇa listens, argues for the importance of his vow, and asks for a way to preserve both his truthfulness and some chance in the battle ahead.
The next day he reports the dream back to Sūrya during worship. Sūrya confirms that the warning was true. Karṇa therefore waits for Indra knowing that the conversation is not a guess or a fear; it is the event he has been told to expect.
Karṇa gives with his eyes open, and that is exactly what makes the gift both admirable and troubling.