What are you doing?
How many times has Leonard said this to Sheldon and he responds with something totally logical to him that no one else can comprehend? Two that come to mind for me:
1) When Leonard mentions the Differential Equation App and Sheldon starts singing "Deep in the Heart of Texas" so Penny can't still their idea. (This was on rerun this weekend)
2) When Sheldon is blowing the 1/4 inch of hair on top of his head to get it out of his eyes.
I expect this could be a long list. LOL.