It's simple.
Wear the shirt indoors, no color. Take it outside and the nearly invisible ink will burst into vibrant colors and turn the smiles on and the thumbs up!

Solar, or UV, activated products like color changing tshirts, tote bags, nail polish, frisbees, and other products change color when exposed to ultraviolet (UV) rays. Sunlight is the best source for the effect, but there are manmade lights, like certain blacklights, that will also enable the color change effect to occur.