When you are adding s to the end of a word, you don't always use an apostrophe (apostrophe being ' ). When you are adding an s because there is more than one thing, you don't use it. For example: The birds flew away The library has lots of books You use the apostrophe s ('s) when you are making a contraction. A contraction is when you put two words together. For example It's a lovely day. I combined the word 'it' and the word 'is' and because of that I add an apostrophe. Here are some more examples: He's going to the store She's a doctor You also use 's when you are saying something belongs to someone. For example: That is my sister's car He took my mom's money I hope this helps!