Just like a mood ring, mood-changing nail polish is meant to change color according to your mood -- for instance, blue means happy, red means angry -- but these products are actually reacting to your body temperature. You can buy mood-changing nail polish, or you can make your own.
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Obtain color-changing ink or dye. If you aren't sure where to look, Matsui has a color-changing ink available, and you can get temperature-reactive Leuco dye from colorchange.com
Measure out a small amount of dye or ink. One-eighth of a teaspoon should be sufficient.
Use the small funnel to add the dye or ink to your clear or white nail polish. Before doing this, you may want to pour out a little nail polish from the bottle so that it won't overflow.
Mix your nail polish with the brush that comes with the bottle. You also could close the bottle and shake it to make sure the color is thoroughly blended.
Add more dye or ink until until you have the color you desire. Add only a little at a time.