Timberlake -- who shares his 4-year-old son, Silas, with wife Jessica Biel -- recently spoke with ET about the challenges of raising a toddler, and Fallon said that, despite being a dad, his experiences were undoubtedly different.
"I have two, I have two girls, he has a boy," Fallon told ET on Sunday of his daughters, 6-year-old Winnie and 4-year-old Frances, while sitting down to promote his most recent children's book, This Is Baby. "I mean Silas, he's a good kid, but he's a boy."
"Even when he was like 2, he was wearing cooler outfits than anyone," Fallon recalled. "I was like, 'Where do you get outfits like that?' He had like baggy pants and like Nikes that are unreleased and stuff. I was like, 'What? This kid is the coolest kid in the world.'"
"I think [for] one of his first words, he was saying 'swag.' He's just so cute," Fallon shared.
So what advice, exactly, does Fallon have for his close friend and frequent comedy collaborator? Essentially, ask someone who knows just a bit more.
"Justin's a good dad and, you know what, when all fails just talk to Jessica," Fallon said, referring to Silas' mom and Timberlake's wife of almost exactly seven years, as the pair celebrated their anniversary on Saturday. "She's awesome, so she'll help!"
Fallon, meanwhile, has lots of fun advice for parents in his latest book about children, This Is Baby.
Aside from becoming a New York Times best-selling writer from his series of children's books, Your Baby's First Word Will Be DADA and Everything Is Mama, Fallon admitted that fatherhood has also impacted him in some unexpected ways -- especially as the father of two.
"I'm much more tired than I've ever been," Fallon joked, explaining how, just that morning, he woke up with both kids and the family dog in the bed with him and his wife, and the kids "just don't care" about the need for sleep.
"But it's the best. I mean, it's the most rewarding thing," Fallon added with a warm smile. "Every day it's something new, and something different."
Fallon's adorable new book, This Is Baby, is available now.