These are Re-Caps for the S1 KID Pro Knee Pad.  They come in a set and are made of High Abrasion PC Plastic.  S1 KID Pro Re-Caps come equipped with high grade male/hook velcro so that you can easily install them to your knee pads. S1 KID Pro Re-Caps are only compatible with the S1 Kid Pro Knee Pads.