how does it make that crinkle sound?

Ok, so I got a request from a person on etsy, to see if I could make my egg cat toys crinkle. She said she didn't want fluff or pellets or cat nip in them just something that makes a crinkle sound.

Now I have never made anything like that but I know that when Moira was a baby she had toys that when you touched parts of them they would make a crinkle sound.

So here is my question to one of you that might now. What is it that they put in there that makes that sound? Like is it a fabric that I can buy?

Thanks for your help!