What is one of your favorite line(s) from the show, The Big Bang Theory?

I absolutely love this show. I will start with one of my favorite scenes/line(s).

Episode: The Infestation Hypothesis

Sheldon knocking on Penny's door: "Penny, Penny, Penny"

Penny opens door 1st time: " What's up buttercup?"

Penny opens door 2nd time: " What's the word, hummingbird?"

Penny opens door 3rd time: " What's the gist, physicist?"

Here is the link : http://www.youtube.com/watch?v=uYrylohza88