Justin Timberlake & Jessica Biel Funny Thing About Marriage ...

12/1/2017 6:57 AM PST

Justin Timberlake and Jessica Biel's Key to a Successful Marriage

This one's gonna hurt, Jimmy Fallon -- but turns out Jessica Biel makes Justin Timberlake laugh his ass off, too ... plus, she's way hotter.

JT and JB looked like a couple of kids on a first date, going all LOL Thursday evening in Brentwood. Yes, they've been married 5 years, and together for more than 10 ... and they're still laughing. See? It CAN be done!

As for what the big joke was? It's possible Jessica and Justin were talking Christmas gift ideas?

Something in a box, perhaps?