Novel and ff ideas for authors

A boy or a girl suddenly got the ability to manipulate his/her body to how heavy it is and it doesn't have a side effect like when you manipulate your hand to as heavy as a car that arm will not fall from your body but will still stick on your body and the owner of this power also doesn't feel heavy when he/she manipulate it but instead it became lighter for him/her, but that is that you can still be killed with only a knife like your durability doesn't change from that of a human.