Book 30, Question 126
How can a drop of dye find a hidden toilet leak?
A toilet can leak slowly into the bowl without any sound, wasting many liters a day.
A grown-up can check by putting a few drops of food coloring in the tank, then waiting about fifteen minutes without flushing. If color appears in the bowl, water is sneaking through a worn part. Fixing the flapper is usually quick and cheap. The test is safe, simple, and saves a lot of water.