A magic mug, also known as a heat changing mug, is a mug that changes colour when it is filled with a hot liquid. This effect is created by using thermochromic ink.
These mugs are often manufactured and sold as memorabilia. For example, a mug may reveal a picture of the town or monument where it was sold. They are sometimes given away to promote organisations. Customers can also have their own photographs printed on the mugs.