It's a nail polish that changes color when you test your drink for date rape drugs. All you do is stick your finger in your drink to see if it changes color. If the nail polish changes color, then there are drugs in your the drink.
Some downfalls I've noticed is the fact of sticking your finger in your drink. Wouldn't that be suspicious, and gross. If I stuck my finger in my drink I'm not sure I'd want to drink my drink after that.