By American English, Genders

Hi, There's something I really don't know: what's the usual, common, informal way to refer to someone whose gender unknown? I know that when the person is not identified we use they (someone, anyone, nobody, etc. - Example: Someone left their book here), and we also use they when speaking in...

Hi There's something I really don't know: what's the usual, common, informal way to refer to someone whose gender unknown? I know that when the person is not identified we use they (someone, anyone, nobody, etc. - Example: Someone left their book here), and we also use they when speaking in...