Better keep track of your valuables with these Nifflers around! When you flip the mug upside down you’ll have to check to see if any valuables fell out. The exterior of this officially licenced 11oz mug transforms from black as hot liquid is added, revealing the hidden image in full vivid colours. The exterior of this officially licenced 11oz mug transforms from black as hot liquid is added, revealing the hidden image in full vivid colours. While this mug may be used in the dishwasher or microwave, for the longest lasting, highest quality product, we strongly recommend against it. PLEASE DO NOT MICROWAVE THIS MUG. HAND WASH ONLY. Due to the need for hot liquid to transform the mug, this mug might not be suitable for young children.