Our walnut tea cups are turned from a solid peice of well, walnut. They are safe for both hot and cold liquids. Slight warping may occur the first time you use it for hot coffee or tea, but don't worry this is completely normal. We recommend you "cure" them on your first use by making a double strength batch of whatever you're drinking and letting it sit in the cup for 24 hours. Traditional methods claim that whiskey mixed in with your favourite hot drink is the best way to go, but whatever floats your boat.