Brainteaser

The solution will appear in the next issue, on stands May 7, 2021.

During a journey, I am usually accompanied by four others. I spend most of my time hidden away in the dark. But if something happens to one of my companions, then I appear. What am I?

Solution from the March 2021 issue: When the two swimmers meet the first time, the combined distance they have swum is the width of the pool, and it has taken them x seconds. Their second encounter takes 3x, and they will have swum a combined three lengths of the pool. Upon their first encounter the slower swimmer had swum 80 feet. Upon their second meeting the slower swimmer will have swum 240 feet (3 x 80 feet) and will be heading back toward the middle of the pool, 40 feet from the south end. So the width of the pool is 200 feet (240 feet minus 40 feet).