A glass green fortress sits on a hill. Certain things can enter, and certain things can't. For example, Beer can go in, but wine cannot. Eyeballs can go in, but eyes cannot. The castle has a floor and walls, but no ceiling. There is a roof, however. What is the rule about things that can go in and out?
I have one, you have one. If you remove the first letter, a bit remains. If you remove the second, bit still remains. After much trying, you might be able to remove the third one also, but it remains. What am I?