From On Christian Doctrine:

In what way did He come but this, The Word was made flesh, and dwelt among us? [John 1:14] Just as when we speak, in order that what we have in our minds may enter through the ear into the mind of the hearer, the word which we have in our hearts becomes an outward sound and is called speech; and yet our thought does not lose itself in the sound, but remains complete in itself, and takes the form of speech without being modified in its own nature by the change: so the Divine Word, though suffering no change of nature, yet became flesh, that He might dwell among us.