Got milk? Pesachim 87A

One of the stranger lines in the erotically charged Song of Songs is (Song of Songs 8:8) "We have a little sister who has no breasts." The Talmud interprets this in a novel way:

Rebbe Yochanan said, 'This is the city of Elam, which merited to learn Torah, but not to teach it. 'I am a wall, and my breasts are like towers.' [Song of Songs 8:10]. Rebbe Yochanan said, 'I am a wall,' refers to Torah. 'My breasts are like towers,' refers to the sages.

Rabbi Varon, who gives a lesson on this page, says that just as a woman's breasts share the life within it, so too teachers of Torah share life with their students. Just as a woman without breasts doesn't nourish other life, so too those Jews who learn but don't share their learning are not nourishing others. Also, a wall offers some protection, but a wall with towers is much more formidable as a means of defense.