Sea To Summit says:

The collapsible Cool Grip X-Mug was built to keep your hands cool when drinking hot liquids. This is made possible by the specially-designed Cool Grip fins that reduce surface temperatures and heat transfer, allowing you to hold your hot tea and coffee firmly without burning your fingers. In addition, the rigid ring keeps the rim of this generously-sized 16 oz /480 ml mug stable. Made from BPA-free, food grade and heat resistant silicone, it’s easy to clean and dishwasher safe.