This Upanishad belongs to the Atharva Veda. As the name of the Upanishad suggests, the teachings revealed in it are such as to leave its students clean, like a shaven head (mundaka), rid of the veil of ignorance. The lucid translation of the text along with Shankaracharya's commentary on it is an additional aid to comprehend the profundity of the subject.
Think always, I am ever-pure, ever-knowing, and ever-free. How I can do anything evil? Can I ever be fooled like ordinary people with the insignificant charms of lust and wealth? Strengthen the mind with such thoughts. This will surely bring real good.
-Swami Vivekananda