Nazir 40 - Real shaving is done with a razor
The Torah said about a nazir that " razor shall not pass over his head ." However, the words "razor," or really "no razor" are already sufficient for us to know the law. Then why are the words "will not pass over his head" written at all? - to include any other way of hair removal, such as pincers or depilatory, and to make it prohibited to the nazir to remove his hair in any way.
But now the word "razor" becomes extra: if in the end the Torah is going to prohibit all kinds of hair removal, then why mention "razor?" Now this teaches an additional law: that when the nazir concludes his term, and shaves is head, that shaving must be done only with a razor. If he uses any other instrument, the shaving does no count, he has to re-do the last thirty days of a nazir, and shave again, this time correctly.
There are in general three categories of people who must shave all their hair: a nazir, a metzora (spiritual leper), and all Levites on the day of their first inauguration for Temple service. The Talmud tries to derive the laws of one of these categories from the others.
Art: Portrait of a Clean-shaven Man by Lucas The Elder Cranach