Why switch expression does not work with float types?

Have you people ever thought of why switch statement does not work with float/double types?
I have read in text books about the constraints of switch statements but i have never thought(infact i didnt take pain to find it out :P) why switch cant be used with floating point values,
but today one of my friend asked me about it(she is doing civil engg)as she has a viva tomorrow on c programming.I was not able to answer her immediately,i searched a lot and i found some answers.I would like to share it with everybody.
The reason is...