Idioms use language metaphorically rather than literally. If you have 'a sweet tooth' it means you like eating chocolate and other sweet things, not that you have a tooth that's made of sugar. Idioms are also fixed groups of words so you can't change the wording of an idiom. For example, you can say you 'make a meal of something' to mean take more time or energy doing something than is necessary but you can't say 'cook a meal of something' or 'make a cake of something'.