"We've known each other for 20 years. I was 19, she was like 14. I did her chemistry homework for her," Kutcher said of acting alongside Kunis. "I think I was her first kiss, like, on the show. We have our first kiss memorialized on a TV show!"
While Kutcher and Kunis clearly have fond memories from that period in their lives, it seems the actual kiss wasn't so great at the time.
"It was really weird. I was like, 'Isn't this illegal?' I don't know, and it was really awkward, because I'm like a 19-year-old kid," he explained. "She was 14! She was like my little sister. I wanted to make sure she was OK."
"She said she didn't think you were particularly good looking when she was 14," Stern interrupted.
"She's lying! She has like, a journal or like, a diary from when she was a kid where she wrote, 'Oh, this guy's hot,'" Kutcher revealed. "She thought I was cute!"
"At first I think she thought that I was good looking, and then shortly thereafter, I was just annoying to her, because I was like a big brother," he clarified. "It was like, 'He's annoying, because he's always looking out for my best interest."
Kutcher eventually romanced Kunis for real, and the pair recently welcomed their second child together, son Dimitri.
See what Kunis told ET about a possible baby No. 3 in the video below.