Myth: 

Your muscles will turn to fat when you stop exercising.

Have you ever had a broken bone, perhaps a broken leg? 

When the doctor removed the cast from your leg, did you see a big gob of fat where your muscles once were?

NO!

The muscles had atrophied, and perhaps there was some loose skin. But  muscle does not turn into fat.

Muscles do have the "use it or lose it" quality, meaning they will waste away if not used.

However, muscle tissue and fat tissue are separate and distinct. One cannot turn into the other.

Perhaps the origin of this popular myth is when someone maintains an active lifestyle and then, upon becoming more sedentary, maintains the same caloric intake. This would result in both muscular atrophy and fat accumulation.