When food is mouth-watering, it looks or smells so good that your mouth starts to make extra spit — you almost want to eat it before it even reaches the table. We use it about meals, cakes, fruit, street food, a photo of food, or the smell coming out of a kitchen. It is a warm, everyday word, a little informal, and you hear it in ads as well as in ordinary talk. People sometimes stretch it to things that are not food, like a mouth-watering price, meaning the offer looks extremely attractive; that use is looser, and many careful speakers still keep the word for food. Though actually, the feeling is the same: something in front of you is so inviting that your body reacts. Before a noun we usually keep the hyphen: mouth-watering soup.