Fad and Thing

Fad noun - A practice or interest that is very popular for a short time.

Thing is a synonym for fad in craze topic. In some cases you can use "Thing" instead a noun "Fad", when it comes to topics like style. informal substitute

Thing noun - A member of the human race.

Fad is a synonym for thing in style topic. You can use "Fad" instead a noun "Thing", if it concerns topics such as idea, current fashion, latest fashion.