Brochas Daf Chof Beis amud aleph

It happened that a certain student (was reciting teachings) before R. Yehuda ben Beseira and stammering. He said to him - open your mouth and let your words shine forth, for words of Torah do not contract tumah, as it is written: "behold my words are like fire, thus says G-d." Just as fire cannot contract tumah, so too words of Torah cannot contract tumah.

According to the Maharsha, the mystical sources tell us that at Sinai, the Torah was shown to us as black fire on white fire. Every letter represented names of HKBH and clearly such a name is the antithesis of tumah. Thus fire does not contract tumah and neither do the words of Torah.