A tiny hole. In a wall. It is IMPOSSIBLE to hurt someone with. Oh, and stuffing someone in it wouldn't work. They would implode before getting there. And the technology for shrinking someone into it has not been invented yet.
oxygen because we breath it, and we grow plants that breath what we breath out, and you can't use it to hurt anyone and its manmade because of the plants we grow. NON-SUGGESTION: This article really helps with my understanding of the universe. Any and all corrections are welcome.