Hanging With Friends will automatically display the last vowel you or your opponent used in a word for free. That's either A, E, I, O, U -- but not Y. Think about this when creating a word. For example, if you've created a verb with, say, an "ed," "er," or "ing" ending, not only will it be easier for your oppontent to find a starting point, but any other instances of the letter e will also be revealed. So try to avoid using the last vowel in your word twice whenever possible.
On the flipside, if the hint feature reveals a vowel in the middle of your opponent's word, you can be certain that there is no other vowel after it. So the word "??I??," for example, cannot possible be "CHIDE." And if the word you're trying to guess has no vowel hint, you know that "Y" is not only in the word, no other vowel appears. Knowing this is crucial to fast guess work. Since few players know this rule, it also gives you a great opportunity to stump them with words that only contain a Y and no vowels as they're likely to try and guess vowels first.
