John Stamos, Henry Cavill, more share knowledge with their younger selves

Are there things you would tell your younger self?

"Good news! One day, you won't have to use T9 to text anymore." Or, "There will be this thing called social media that will let you see what everyone is up to without ever actually having to talk to them, and it's genius."

Yup, those are things we would tell our former Justin Timberlake-lovin', kitten heel-wearing selves. And, celebs have lessons that they've learned too. In a bit called A Message to My Younger Self on Late Night with Seth Meyers, some stars recorded shout-outs with some very valuable info.

It's hard to choose, but our favorites have to be from the stars of The Americans and Grandfathered.

Keri Russell

"Whatever you do, don't cut your hair short during the second season of Felicity. No, I'm serious. People will freak the hell out. You'll get hate mail. You'll even get death threats. But gradually your hair will grow back, and your fans will forgive you. But you will never, and I repeat never, forgive your fans."

John Stamos

"On your 18th birthday, the Prince of Darkness, Satan, will offer you eternal youth in exchange for your soul. Little John, I'm telling you right now, you take that deal son."

Click play to see what hilarious knowledge Henry Cavill and John Goodman dropped on their former selves.