The document discusses the history and structure of limericks. It notes that limericks gained popularity in the 1800s with Edward Lear's Book of Nonsense. A typical limerick follows an AABBA rhyme scheme, with the first, second and last lines rhyming and having the same number of syllables, and the third and fourth lines rhyming and also having the same number of syllables. The document provides examples of limericks that demonstrate this structure.