What is difference between "on a diet" and "in a diet"?

I know that "on a diet" is correct expression.

But, I want to know the cause that "in a diet" is not correct.

And I wonder why "in a hurry" is correct and "on a hurry" is not correct.

Both "on" and "in" have as meaning of state, don't they?

  • 2
    Some prepositional phrases are simply idiomatic. On a diet and in a hurry are great examples. Others would include: on the phone, on hold, on a mission, on fire, in a pinch, and in cold blood. – J.R. Nov 23 '16 at 10:29

No doubt, some dictionaries say that you can use these prepositions to refer to a state, but they also say that you use the preposition "on" to refer to something that you are using, eating, or drinking. So you use "on", not "in", for examples:

  1. I am on a diet.
  2. He is on antibiotics.
  3. He is on drugs.
  4. He is on the phone.
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