At a glance it looks like a regular black mug with the word ‘OFF’, nothing extraordinary about it, but once you add a hot beverage, the magic begins..... (via ThisNext Blog)

I probably should have some kind of warning signal that tells people if I have or have not had my coffee today. This is a good start. It says OFF in big white letters, until you pour hot liquids in it and it changes to say ON in black letters on a white background! Ah, the wonders of heat sensitive mug paint.

When filled, the mug color changes to white and says ON.

On/Off is perfect in our house with one tea addict and the other a coffee addict. I may have to purchase.

Love this!!!

I HAVE TRIED THESE BEFORE

THEY ARE REAL NICE

A MIND OF ITS OWN

TRUST ME, THEY WORK

Simple ideas are sometimes the best. The On Off Mug is a standard gift-shop mug with a little something extra: a thermo-something chemical paint coating.

When room temperature, the mug is a stark black color with the word OFF displayed (is that Helvetica? Nice touch). When you pour a hot liquid in the cup, it gradually changes to a relatively white color with the word ON shown.

Just a great, fun piece of design. It's also nice and handy in letting you know when your drink gets cold, so you can have your assistant get you another cup.

This coffee mug features heat sensitive material which will warn a person if the mug is filled with either cold or hot liquid. Great mug for the office.

very unusal