Kiss Nail Artist stencil help?

Hi, I bought the "Kiss Nail Artist" set with Black, White, and silver thin brushes and 30 stencils. I was wondering if you can reuse the stencils? If so, don't they lose stickiness, and they wont stay on the nail and the polish will run underneath? Also, in the stencil plastic packet, there's this weird square with one sticky side, what is it?