My son rolls to his belly when he sleeps now?

My daughter did that to. she was 6months at the time, i got scared when she did that so i would turn her on her back but then like 5 in later she turn over to her tummy. so i was told to not worry because she knows how to move her neck and if i do worry just to check up on her and make sure shes ok...shes now 3 and loves to sleep on her belly, so i think shes a tummy sleeper lol.