A young boy needed to pee while he was swimming, and thinking no-one would see, he did so in the water. He acted like nothing was different, but before long the rest of the swimmers were glaring at him. He looked down and saw that the lower half of his body was painted red.
It turned out the pool water had a special chemical in it that changed colour when it came into contact with urine. It was clear to the other swimmers what the boy had done. The boy panicked and fled from the pool.
It’s important to note, however, that making such a chemical compound is extremely difficult. If such a chemical really was added to pool water, then people wouldn’t be able to swim in it.