One testicle too many..

Yesterday my oldest son and I were talking about surgeries. Of all things.. But I was telling him about the surgeries I had had on my jaw when I was a teenager, and then we talked about his hernia surgery when he was 2. He asked me how I knew that he had a hernia, and I told him about the time he came inside from playing in his kiddie pool, and when I changed him, one of his testicles were swollen up like a balloon.
His response "I used to have 2 testicles??"