Yeah, but a roo's what? ;) I've been reading Eats, Shoots & Leaves
by Lynne Truss and it's empowered me to embrace my love of pedantery and I want to explain a few of the basic rules about apostrophes. You see, if you want to say the plural of something by putting an "s" on the end, it doesn't need an apostrophe, eg "There are lots of kangaroos in the paddock today." If you want to designate ownership, then you should put an apostrophe before the s, eg "Look at the kangaroo's pouch." The exception is the word "it", so when you say that it owns something, you don't need the apostrophe, eg "The kangaroo has a joey in its pouch." The apostrophe is also used when you want to indicate that you're skipping one or more letters, like when you say "you're" instead of "you are".
These rules aren't difficult to learn and I'm pretty sure I've told you this before. I don't say this to embarass you, but because it jars with me every time I see the apostrophe misused.