When my brother and I were little and sitting in the car for a long trip on rainy days, we followed the rain drops running down the car windows. Sometimes we chose each for one drop and waited excited which one will be the first to reach the bottom of the frame first - which one will make the race.
Since rain drops look all alike, it was sometimes hard to tell who won.
15 years later, I designed stickers with color pigments embed that can be attached to your window on rainy days.
Which drop will win the druppel race??