The silicone strip helps to keep socks on at all times and can be found in all sizes except 28/30 and 31/33. Material: 90% cotton, 8% polyamide, 2% elastane, 100% pure silicone inside the heel Size: follow the child's shoe size firstly, not the age indication Toe: "Seamless toe" with a flat seam to not irritate small and sensitive toes. Certificates: STANDARD 100 by OEKO-TEX® What is OEKO-TEX®? All STUCKIES® socks are certified according to STANDARD 100 by OEKO-TEX® - one of the world's best-known labels for textiles tested for harmful substances. Every thread in our socks have been tested for harmful substances and that the socks are therefore harmless for human health.