The Big Bang Theory brought back one of its funniest recurring bits in "The Separation Agitation" — Fun with Flags, Sheldon (Jim Parsons) and Amy's (Mayim Bialik) vlog about the flags of the world ("you gotta know how to hold 'em, you gotta know how to fold 'em").

Fun with Flags appeared one time previously in Season 10, in "The Veracity Elasticity," which introduced the house band Footprints on the Moon, with Howard (Simon Helberg) on keyboard and Raj (Kunal Nayyar) on guitar. But the band really came into its own in "The Separation Agitation" with the theme song for "Fun with Flags: Behind the Flags: A Retrospective"

The song goes:

"Wondering how it all began?/ You'll need a good attention span/ For information and entertainment/That's equally effective/It's Fun with Flags/Behind the Flags/A Retrospective/Flags!

It was a call-in show without callers until Bert (Brian Posehn) called to tell them he got a girlfriend.

Bert's gold-digging girlfriend and Howard, Bernadette (Melissa Rauch) and Stuart (Kevin Sussman) trying to cope with sending baby Halley to daycare were the main plots of the episode, and both had their funny moments. I particularly liked Sheldon being sent to time-out after saying mean things to Bert and his girlfriend and Howard's admission that Halley should grow up more independent than he is ("It wasn't until college that I learned you can put a thermometer in your mouth").

But the star of the show was Fun with Flags. The retrospective was so successful that Sheldon and Amy brought it back at the end of the show for "Fun with Flags: Behind the Behind the Flags: A Retrospective Retrospective."

Depending on how Young Sheldon does, maybe there could be a Fun with Flags spin-off in the future!

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