At one time, pride entered into Brahma. He said, ‘since I am all fire, I can swallow everything. So now let me swallow everything and prove that I am the greatest of the cosmic gods. Who can dare to stand against me? I can eat fire! I am all fire! I am Brahma!”
When Vishnu saw the extent of Brahma's pride, he took the form of a little boy, four or five years old. This little boy came before Brahma holding a dry reed in his hand.
The little boy said, “Lord Brahma, you are the greatest of all the gods. Here is a dry reed. Let me see you burn it or swallow it.”
Brahma immediately engulfed the little boy with flames. The flames wanted to devour not only the reed but also the little boy. But although the flames burned intensely, they could not swallow the little boy and they did not even touch the dry reed in his hand.
“How can this be?” cried Brahma. “I can burn up everything and everybody, but a single dry reed and one little boy are beyond my capacity. I do not understand it.”
Then Vishnu took his real form and said to Brahma, “This is all because of your pride. I wanted to smash your pride by showing you that you could not do anything to a reed. Now you should not boast anymore. You have to remain silent.”
So this was how Brahma was humiliated by Vishnu.
from The Earth-Illumination-Trumpets of Divinity's Home (Stories from the Indian Scriptures: the Puranas)
by Sri Chinmoy