Answer from Alicia Wassenberg: No, your muscle doesn't turn to fat, but if you don't workout your muscles will start to shrink and get smaller (atrophy). When you start working out again they will build back up again.

Answer from Ken Moody: I think this idea may have initially come from bodybuilding. Because we saw bodybuilders in their off season still looking vaguely V-shaped, but with a lot more body fat than during competitions. So it actually LOOKED like muscle had TURNED into fat! What really happened is the muscles atrophied (got smaller) and the bodybuilder gained fat because he/she was OFF the strict diet of the competitive season! But it is physically impossible for muscle to turn into fat.

Answer from Tami Racaniello: Muscle cells and fat cells are completely different, so one cannot turn into the other. You can shrink fat cells and build muscle cells. When you don't work out, it may seem like the muscle turns to fat, because the muscle cells will shrink, and the fat cells expand.