When Leonard isn't able to fly out to his old high school to give a graduation speech, Penny helps him give the speech via Skype instead. Unfortunately, she was only able to get him a cap and gown from a costume store, but Leonard wears it well in this funny clip.

Sheldon and Leonard's moms meet for the first time, for one of the funniest episodes of the season. Laurie Metcalf and Christine Baranski guest star, and their characters butt heads immediately with Penny caught in the middle.

Okay, so not a funny moment in the slightest, but this one is important. Amy has had enough of Sheldon treating her badly, so she finally tells him they need to take a break. When they end their call, however, we see that Sheldon has an engagement ring for her. And that's how the season ends. Heartbreaking!