John Stamos and Britney Spears Get Their Gleek On

Actor John Stamos is making a guest appearance on the hit FOX show "Glee" -- and he'll be on the Britney Spears episode!

As he makes the "loser" symbol on his forehead, Stamos laughs, "This is the first time I've ever done this. Which hand is it? No, it's this hand!"

Everything thing you need to know about "Glee"!

Uncle Jesse talked with "Extra's" Jerry Penacoli Thursday in Long Beach, where the actor was celebrating his 47th birthday at the Summer Concert Series.

"Everybody was buzzing about Britney yesterday and how nice she was and how she gets the comedy of that show and what they wrote for her," he said. Stamos is playing a dentist on the show (to air Sept. 28). "My hands are too big for a dentist, aren't they? My dentist has little tiny hands. He sticks them all the way down my..."

The "Glee" stars... before they were famous!

What are Stamos' dental duties on the show? "The kids with bad plaque on their teeth come in and I put 'em under," which then launches them into Britney dream sequences," he revealed.

As for turning 47, Stamos admitted, "This is really the happiest I've been in a long, long time. I feel very settled and focused."