The 'Abbott Elementary' EPs talk jokes cut from 'Always Sunny' crossover, including backstories for Frank and Mr. Johnson.

Here's some background and inside jokes to help fill in some of the gaps for viewers who don't watch both Philly-based shows.

Lisa Kudrow revealed during an interview on “The Drew Barrymore Show” (via People) that she recently discovered a note the late Matthew Perry left for her inside the “Friends” cookie jar, which he gifted her after they wrapped filming the series finale in January 2004. Kudrow wouldn’t reveal what the letter said but mused: “Timing …