Explain SMBC — the wiki for Saturday Morning Breakfast Cereal

This comic plays on the awkwardness of "the talk" -- the traditional parental conversation about sex and reproduction. A mother is sitting on a bed with her daughter, holding a book titled "Your Body," and realizes mid-conversation that it would actually be more efficient to list the things that don't ever go into or come out of vaginas, since that list would apparently be shorter.

The caption below adds that the mom then "pondered for 5 minutes, stood up, said 'good luck,' and left." The joke is twofold: first, it humorously implies that the list of things associated with vaginas (in terms of biological processes, medical procedures, and sexual activity) is so extensive that describing it by exclusion would be simpler. Second, the mother's inability to even come up with the shorter list, followed by her giving up entirely, suggests the topic is so vast and complicated that parental guidance is essentially futile. It is a characteristically SMBC escalation where the initial joke gets pushed to an even more absurd conclusion in the bonus text.