Challenge Mode encourages you to get creative by not allowing you to use the same words in three separate trials. If you are feeling uncreative, you can take advantage of The Great Adjective Exploit to bypass this entirely: just add an adjective (or gibberish) in front of a word and the game will give you a free pass to use it again. For instance, to "reuse" MONSTER, type SCARY MONSTER or GIANT MONSTER and the game will react as if you just used the term MONSTER for the first time!