One day while travelling with a companion, Nanak took shelter in the house of a poor, low-caste Hindu carpenter named Lalo. He took a liking to Lalo and stayed with him for two weeks. Then he heard that people were gossiping. They said, "Nanak is a high-caste Hindu; why should he be staying with a low-caste man? It is not proper."
One day a wealthy landlord of the neighborhood decided to give a big feast and to invite all the four castes of Hindus -- brahmins, military, merchants and manual laborers. A brahmin friend of Guru Nanak came to him and told him about the feast. "You really must go," he said. But Nanak did not believe in castes, and considered all men equal. He did not like the idea, and said, "I do not belong to any of the four castes, so why invite me?" "Ah," said the brahmin, "now I see why people call you a 'heretic'. Malik, the landlord, will be very displeased with you if you refuse his invitation." And he walked away.
Nanak did not go to the feast, and, sure enough, afterwards Malik came and confronted him. "Why did you dishonor me by staying away?" "Well," replied Nanak, "I do not crave fine food. But if this offends you, then I will eat some of your food." But Malik was still not happy, and accused Nanak of ignoring his own caste and eating and staying with Lalo, a low-caste man.
"Then give me my share of elegant food from your banquet," said Nanak," and turning to Lalo he asked him to bring him something from his stock of simple food. When both foods were set before Guru Nanak, he took Lalo's coarse food in his right hand and Malik's fine food in his left, and squeezed them both. Lo and behold, from Lalo's food milk flowed out, and from Malik's, blood!