Here's what you need to remember: it's with an apostrophe means 'it is' or 'it has'. In the sentence "It’s a good idea" it works just like an ordinary contraction. But its (without an apostrophe ...

As the famously nitpicking former editor Lynne Truss once put it, “To those who care about punctuation, a sentence such as ...

who says she received "hundreds" of messages on the apostrophe society after its closure, most proclaiming its lasting importance. The Apostrophe Society reported a 600-fold increase in demand ...