Q: How did the ankylosaurus use the plates on its back?
A: Just because you have something doesn't mean you use it. For instance you have an appendix that's not good for much of anything. But ankylosaur plates were probably very good for blocking meat-eaters from attacking it. It was a low, squat animal so like a porcupine, it could probably protect its underbelly by being so hard to turn over and so tough and pointy on the top.
Q: Did the ankylosaurus have the hardest shell?
A: Ankylosaurs had hard shells, even on their eyelids! Hard to say which was hardest of any dinosaur though.
Q: If one of the plates on the back of stegosaurus broke off, would it have grown back?
A: I don't think bony plates would grow back on a stegosaurus. Bones do heal over, and there would probably be a hard lump around the broken plate. We find dinosaurs with healed leg and rib fractures with big lumps of bone around the break. The stegosaurus would probably have had arthritis and maybe a backache. No kidding.
Q: Did the velociraptor really have a retractable claw like a cat?
A: Velociraptor could sweep its claw up and down in an arc, but it wasn't really retractable into its flesh like a cat's. It was pulled up and snapped down with the aid of muscles though, and was a nasty weapon.