Is it its or it's?

One of the weirdest situations when using an apostrophe is the decision between its and it's. The first is a possessive and the second is a simple contraction -- a combination of the words it and is. If we want to combine the words do and not into a contraction, we write the word don't. Easy enough. So when we want to combine it and is into a contraction, we write it's: It+apostrophe+s. The weird situation is that sometimes we want to write a possessive using the same letters: its. And we know that possessives use an apostrophe: Bob's. So we assume the same case here! But no, the truth is just the opposite. If I write "Bob's car quit when its engine died," I will put the possessive apostrophe in Bob's car but I don't put the possessive apostrophe in its engine. If I wrote it's engine, that would mean it is engine which would make the sentence sound really silly. So, the thing to remember is that when it comes to possessives, its leaves off its apostrophe.