Shhhhhh….. let it be secret until your loved ones or clients pour some hot coffee or water or milk in the mug. This mug appears fully black unless you pour hot liquid. The photo or anything else which is printed on this appears when you pour hot liquid and starts disappearing when the liquid gets colder.
It’s also an indication that the temperature of the liquid inside is falling and that is good to drink.
Everyone who receives this surprise, loves this!! What else do you need.