During the kurukhetra battle, one day Gandhari asked her son Duryodhana to come and meet her naked. Durodhana agreed and when he was on his way, Krishna stopped him and advised not to go naked infront of her mother, that would be a shameful act.
Duryodhana thought its a true advice and wrapped him with a Banana tree leaf. Gandhari removes her bandage and was surprised to see that her son was not completely naked as it was instructed to him.
Duryodhan's body turned into iron except the part which was covered by banana tree leaf and because of this, he was defeated in wrestling (mallyudha) against Bhim.
So the question is, how Gandhari knew that her eye sight/rays (when no bandage) had so much power that it could make a person's body turn into an iron?