Harry Potter heat changing mug, Cold the mug shows the famous Expecto Patronum charm which wizards and wizards can cast when they need special protection from Dementors. When hot liquid is poured in, the mug transforms to reveal Harry's stag Patronus on the front and a Dementor watching from the sky on the back. Once the drink is finished the mug returns to its original state as it cools. An essential for any Harry Potter display.