The Truth about "It's"

More than a few people confuse "it's" (for "it is") with "its" (the possessive pronoun) in their writing. What I didn't know was the history of "it's". Daily Writing Tips provides this interesting tidbit:

when the third person neuter possessive adjective came into the language in the 16th century, it was spelled it’s for the very reason that the new form was modeled on the ’s of the possessive noun. The spelling it’s for the possessive adjective was acceptable “down to about 1800 (A.C. Baugh, A History of the English Language, p. 295).

Although we generally have to teach standard forms, it's good to keep in mind that what is "incorrect" today may have been "correct" yesterday.