May I, dare I, presume to correct some of my fellow contributors? I was brought up at a time when bad spelling or misuse of words brought about a sharp rap on the knuckles. This one was my primary school teacher's particular bete noire.
It's with an apostrophe means 'IT IS'. It does not indicate a possessive. It's the one exception (see what I did there?).
Its without the apostrophe does indicate the possessive 'the car finished up on its roof' is right.
Probably shouldn't identify worst offenders, so I won't. You know who you are....
The rules are not confusing, it's just that they are often very badly taught and frequently incomplete. For example, 'I before E, except after C'. It should continue to say, "when it sounds as 'ee'. To that rule there is only one exception, 'seize', and that only because it is a corrupt spelling of a French word.
If you say weird slowly it is actually two syllables, so does not sound as pure "ee" and thus falls outside the remit of the rule. The same applies to weir. It is not pronounced "weer".
'Weird' isn't two syllables, that's really dodgy elocution, you end up saying 'Weee /urd'-when properly ( or at least with RP- I like dialects so not critisising them) it's more 'weard'.