The essential rule to remember about the word "it's" is that the apostrophe indicates a contraction for either "it is" or "it has." When the term does not convey these meanings, the correct form to use is "its," which signifies possession. This distinction is straightforward, yet many people still confuse the two, leading to significant errors in punctuation.

Lynne Truss, in her book "Eats, Shoots & Leaves," emphasizes the severity of these mistakes, suggesting that even those with advanced education can fall victim to this common error. Misusing "it's" instead of "its" is presented as a serious grammatical lapse, deserving of severe consequences in a humorous and exaggerated tone. The statement serves to underscore the importance of mastering proper punctuation for clear communication.