Mouth
noun
When a vocab is made up of a single kanji with no okurigana (hiragana attached to the kanji) it's generally going to have the same meaning as the kanji it came from. From that, you know this word's meaning is mouth.
くち
When a vocab is made up of a single kanji with no okurigana (hiragana attached to the kanji) it's generally going to use the kun'yomi reading, which you did not learn from learning the kanji. To remember this word, you should use a mnemonic. With your mouth, make a "kuchi kuchi kou" (くち) sound. Go ahead, make it. Focus on your mouth and try to imagine each shape it makes while zooming in on it. You're talking to a baby, which is why you're making all these ridiculous noises. Also, instead of "coo" at the end, you're saying "kou," because that's the reading of the kanji (こう). くちくちこう！くちくちこう！ Sounds like something from Arrested Development, actually.
口にソースがついていますよ。
There is some sauce on your mouth.
これを口に入れてください。
Please put this in your mouth.
このペットボトルは口が大きすぎる。
The mouth of this plastic bottle is too large.
ちゃんと口のケアをしないと、女にモテないだろ。
If you don't take good care of your mouth, you won't have much luck with women.