One evening, I agreed to babysit my best friend Kelly’s 8-year-old son, Thomas, while she attended a work retreat. My husband Ryan grumbled but relented. I’d always admired Kelly, a single mom since age 16, and saw this as practice for our own future child.

Thomas was a delight—games, dinner, movie—until he fell asleep. Carrying him to bed, his shirt rode up, revealing a birthmark identical to Ryan’s. Then I noticed his nose, chin… everything echoed my husband. Heart racing, I pocketed Thomas’s ice-cream spoon for DNA.

Ryan had acted strangely about the babysitting; Kelly had asked about his reaction. Suspicion gnawed. I collected Ryan’s hair and rushed a paternity test. A week of dread later: 99.9% match. Ryan was Thomas’s father.