The main thing about having a TV show with flashbacks, flashforwards, or even a slow-moving plot, is you that can't have actors aging rapidly throughout the course of what's supposed to be a certain amount of time—let alone the course of one conversation. Considering this was the conundrum that How I Met Your Mother faced when the show premiered all the way back in 2005, producers had to get creative to ensure that their teenaged actors playing Ted's kids—Lyndsy Fonseca and David Henrie—didn't look mid-twenties when a lengthy conversation with their father was over.

So, smartly, the filmmakers decided to film series finale scenes with the kids all the way back in 2005, and save them for the end of the show.

People has the deets:

"We shot this last little section nine years ago," actress Lyndsy Fonseca tells PEOPLE. "It was so long ago, I don't really remember what I said."

Although Fonseca has appeared throughout the series in flash-forwards as the daughter of the titular Mother, all of her scenes were shot during the CBS sitcom's first season – the only way to ensure her on-screen brother, David Henrie, looked the same age for the entire series (despite the fact that he was going through puberty at the time).

"David was growing so fast. He grew like four shoe sizes and his voice dropped, so it was very apparent," says Fonseca, 27, who filmed improvised scenes and generic reaction shots with Henrie that could be used for any occasion throughout the course of the series.

"When we started, I wasn't even old enough to drive," adds Henrie, 24. "Now, I just bought my first house. I come home … and my cousins are watching the show. It is always a funny feeling. They always say, 'Baby David is on TV!' "

Those poor kids. Listening to their dad babble on about God knows what for nine years. They missed college! 

[via People]