Raven raised her hand. "Both Boggarts and Dementors are pretty good at terrifying people. One turns into what you fear more than anything else, and the other drains all your happiness no matter what. But if you know that something is a boggart, then it wouldn't scare you as much as if you didn't know. On the other hand, Dementors steal your happiness even if you have been through it many times befre. Only the Patronus can help you." The professor hadn't asked her what the differences were, but she decided to say them anyway.