Sheldon tells her that that’s for things he never tells anyone, including her.
Well, Amy is very intrigued by the idea that Sheldon keeps some things secret from her and begs to hear one thing he hasn’t told her.
That’s when Sheldon tells her that he actually got his driver’s license two years ago.
After they get back to the apartment and Amy retires to the bedroom to get away from Sheldon, he, realizing that he can’t quite tell who should know what about their romance, makes a chart to try keeping things straight.