Don't explain why, just make her pay to have them sharpened. It's amazing how quickly our children learn when things directly impact THEM. They are your sewing scissors and there is no explanation required. Falls into the same catagory as when my teen daughter decided that the pearl earrings my parents gave me for my 21st birthday would look good on her and she lost them. Took them without asking. It took her a while to replace them. They now have double emotional value for me.