To those who would assert that "it's" is two words:

I hope you boys aren't designing a word processor or spell checker.
Double-click on the following and see what gets highlighted:

it's

Mr. Webster say's it's been a single word since circa 1555... in fact
it's listed in every dictionary I've checked as a contraction of "it
is" or "it has". As a single word, its tense will change with context
unlike its derivatives. BTW, a contraction is "a word":

contraction (kún-trak'shún) n.
1. The act of contracting or the state of being contracted.
2. a. A word, as won't from will not, or phrase, as o'clock from of
the clock, formed by omitting or combining some of the sounds of a
longer phrase. b. The formation of such a word.