I agree with Sas. You're taking a real chance letting your cats play with string.

I had an incident last year when I was sewing some kitty pillows for an adoption day/tag sale. I didn't clean up all the thread around my apartment and didn't think anything of it. My baby Marina Mar (Rainbow Bridge 11/01) ate some and we wound up at the emergency vet. $75 later, I learned a lesson.

Now I buy them toys. If I DO give them something other than a cat toy to play with, I make sure that it's too big to swallow. But it's rare. Cat toys are the safest.