Alphonso and his sister Sadie are entering the airport to meet their mother when Sadie gasps in surprise and says to her brother, “You see that man in the crowd over there?”
Alphonso replies, “It’s Bernie. I don’t believe it. Let’s go introduce ourselves.”
Neither of them had ever met Bernie before. They’d never seen a picture or painting of him, nor was he a famous celebrity.
Alphonso was born as a twin, but he was separated from his twin brother at birth. Bernie is Alphonso’s identical twin, which is how Sadie was able to recognize him in the crowd.
This is kind of a cheat, because there are probably other possible explanations. But we can’t have everything :)
Imagine an HIV test that is 95% accurate (false positive rate of 5%) and around 2% of the tested population is infected with HIV. What is the probability that you actually have HIV when your test comes back positive?