Anthony G Williams
Greybeard
Actually it's "it's".But still its no excuse.
With most words, the apostrophe is used before an "s" to indicate "belonging": so it's "Johnny's bike", because the bike belongs to Johnny. You have to be careful over words which end with "s" (such as when the word is a plural noun) as the extra "s" is often missed off, as in "the inhabitants' bikes".
"Its/it's" is the exception to the "belonging rule". Here, the apostrophe is only used to indicate that words have been shortened and run together. So you write "it's" instead of "it is" - but not in any other circumstances. So it would be "it's too late" but "the bike had its own place". This is very problematic for native English speakers, who often get it wrong. The apostrophe is used in other circumstances where words are run together ("he's late again" for "he is late"). It also used to be used in the middle of a word to indicate that the word had been shortened, but this is now uncommon and best ignored.