We could've gone with "Zach Galifianakis' Sons' Testicles Fit in His Belly Button", but the Tonight Show's official YouTube page already ran with that.
During his interview with Fallon, his first public appearance in over a year, he discusses how his son's testicles do indeed fit into his belly button (that being Zach's belly button; there would be something terribly wrong if his son - 'Sheila One' - could fit them into his own belly button). He also divulges two of his son's myriad of nonexistent names after a fashion, before going on to endorse "prenatal spanking" as a form of "discipline." In fact, the only thing he doesn't really skirt over humorously is his notable weight loss. See, that's what having a kid does to you...
According to Us Magazine, this is his first child with his wife of two years, Quinn Lundberg.
For those wondering about the weight loss, it was required for his latest movie Birdman. When asked by a red carpet pundit about his weight loss, his simple response was "I'm dying." When she just banged on about how great he looks in his suit, he mumbles it's "a ladies size 50) before abruptly wandering off. See the video over yonder.