Explain SMBC — the wiki for Saturday Morning Breakfast Cereal

The Joke

The comic presents a limerick written on a yellow background:

"This limerick goes in reverse
Unless I'm remiss
The neat thing is this:
If you start from the bottom-most verse
This limerick's not any worse"

The joke is that the limerick claims to work equally well when read in reverse order (from bottom to top). Reading it backwards produces: "This limerick's not any worse / If you start from the bottom-most verse / The neat thing is this: / Unless I'm remiss / This limerick goes in reverse" -- which is itself a coherent limerick with proper AABBA rhyme scheme.

The Humor

The humor is in the clever construction of a palindromic (or more precisely, reversible) limerick. Limericks follow a strict AABBA rhyme scheme, where lines 1, 2, and 5 rhyme with each other, and lines 3 and 4 rhyme with each other. This limerick is self-referential: it describes its own property of being reversible. When you read the five lines from bottom to top, the rhyme scheme still works (AABBA) and the meaning still makes sense -- in fact, the reversed version describes the same property. It is a piece of constrained writing that is both a limerick and a meta-commentary on its own structure, showcasing Zach Weinersmith's fondness for wordplay and mathematical/structural humor.

References

• Limerick -- A form of humorous verse consisting of five lines with an AABBA rhyme scheme, where lines 1, 2, and 5 are longer and lines 3 and 4 are shorter. The form originated in the early 18th century and was popularized by Edward Lear.
• Palindromic/reversible poetry -- A form of constrained writing where a poem can be read in both forward and reverse order while maintaining coherence.