Well, when my son was little, and we used to play the alphabet game in the car while on a long drive (where you call out names of things, one for each letter, as soon as you see them, in alphabetical order) we used to say that when you came to X, you could use words that start with "ex-".

Exhaust ...

Actually, come to think of it, there aren't that many nouns that start with "ex-" either. Most are either verbs or adjectives. So I guess you have to allow noun phrases, i.e. adjective + noun.