"It was a Passed Ball"

My 3 1/2 year old boy just corrected my baseball viewing yesterday. We were watching the Red Sox and Orioles for a few mintues. I caught the very end of a pitch as it scooted under the catchers mit and through to the screen. I also noticed the batter in the last part of a swing. This all equaled a foul-ball to me. My son said, "That was a passed ball, Daddy." I told him that it was actually a foul ball. He promptly told me, "No, the batter didn't hit it so that means it was a passed ball." The replay revealed that he was right, except that there were no runners on and therefore no advance on the play, but we'll leave the finer details for later.