It's Always Sunny In Philadelphia: 10 Best Recurring Jokes

So, it looks like Charlie isn't the only one dealing with some psychological troubles.

Frank Reynolds is a perplexing man, as his loaded bank account means you'd expect him to live a life of luxury, but he instead chooses to live in squalor, sharing a bed with his best friend and eating cat food just to get to sleep. It's clear that Frank is dealing with a lot of stuff, but he seems happy; so long as you don't call him "donkey brained."

This is a joke that's turned up many times in the show, most notably in "Reynolds vs Reynolds: The Cereal Defence" when Frank provided a certificate to certify that he did not have "donkey brains". This allegedly came after he was sent to a "nit-wit" school and wanted his mother to get him proof that he was no longer mentally unstable.

Of course, Frank is always oblivious to how ridiculous he sounds, and anytime he brings up his "donkey brains" - or lack thereof - it gets a big laugh. But, more than that, this recurring joke even became a fundamental piece of Frank's backstory, as we later found out in the episode "Psycho Pete Returns" that he did spend a lot of time in a mental health ward.

By his own words, you could easily say that he had "donkey brains" in his youth.