"Catching a cold" is an idiom - it means to come down with the symptoms of the cold virus.
A common expression or idiom
idiom means expression like a page in a book
Rolf Harris This expression is not an idiom.
Idiomatic expression
"You" is not an idiom. It is a pronoun.
no an idiom would be like "it's raining cats and dogs"
Quit bugging you-an expression.
This is not an idiom. It means exactiy what it says - "advanced" means moved up or moved along, so the person is older.
Idiom
This is not an idiom. When you see AS ___ AS ___ you are dealing with A Simile. They are comparing the temperature to a witch's supposedly cold body.
An idiom is an expression, the meaning of which is dependent on cultural context and social understanding. The meaning of the idiom is not predictable based on its constituent elements, but is merely an expression. An example of an idiom is "kicked the bucket". These words are not taken as literal, but as an idiomatic expression.
It is not an idiom. Unkindness is often called heartlessness, and so the expression "have a heart" means "Do not be unkind."