This is why I would never use an old, used sheet. I have had several good quality, 100% cotton sheets rip across the top as I pulled them up to make our bed. The fabric was not worn-looking. The sheets were not that old, but the fabric just shredded - across the top on the side my husband sleeps on, not on mine. The answer - he sleeps without a pajama top, and the oil from his skin had weakened the fabric. It looked just fine until tension was put on it. Then it just pulled apart. If I were thinking about using an old sheet, I'd test it's strength carefully before I did. I'm sure many of them are fine - just don't use any of our old ones. LOL