Minecraft wiki entry for Damage, associated text: When a player or mob deals melee or projectile damage to a player or mob that is wearing Thorns-enchanted armor, part of the damage is reflected back at the attacker. The amount varies with the enchantment level. (Level * 15% chance of inflicting 1–4 damage, or Level - 10 damage if level is over 10) however, thorns increases the amount of durability lost per hit on the enchanted armor.