In your elementary French class, you learn the word for mouth: la bouche. And this word is feminine. It's la bouche, not le bouche.
However, the expression "word of mouth" is masculine: le bouche à oreille.
After you've been saying la bouche for years, you will be tempted to say "la bouche à l'oreille." But the expression is in fact le bouche à oreille.
Another expression where bouche seems to suddenly become masculine is faire du bouche-à-bouche à quelqu'un: to give someone mouth-to-mouth resuscitation.
In most other idioms, though, bouche remains feminine (as you would expect):
parler la bouche pleine = to talk with your mouth full
avoir la bouche amère = to have a bitter taste in your mouth
bouche cousue ! = mum's the word
et pour la bonne bouche = and last but by no means least
Another basic feminine word is la feuille (the leaf). But compounds that use this word can be masculine. For example, the pastry known in the US as a "napoleon" is le millefeuille in French.
Portefeuille, meaning "wallet" or "portfolio" is also masculine: le portefeuille.
On the other hand, the plant known as milfoil or yarrow is la millefeuille.