David Letterman's joke went right over Reese Witherspoon's head when she showed up on Tuesday's "Late Show."
Jokingly insinuating that he was Ava's real father (as opposed to Reese's ex-husband Ryan Phillippe), Letterman responded, "Well, that's an interesting coincidence."
The audience started laughing right away at the joke but it took Reese a minute to catch on to the quip. "Oh, my God! No, no, no!" she exclaimed once she got what Letterman was saying.
"I got it now. ...I was having a blonde moment," the "Legally Blonde" actress explained. "I have a lot of them lately."
Letterman added, "That was the farthest thing from your mind!"
Steering the conversation back towards her daughter, Reese shared that Ava has expressed an interest in acting. "She's very artistic though," she said. "She's sort of a songwriter and she paints. She's wonderful."
According to her mom, Ava has yet to run with the wrong crowd. "Not yet," the 39-year-old Oscar winner answered when asked if her teenager is rebellious. "No smoking or drinking."