Penny: (on Leonard's parents not considering his birthday as an "achievement" to be celebrated) That's so silly.
Sheldon: It's actually based on very sound theories; his mother published a paper on it.
Penny: What was it called, "I Hate My Son and That's Why He Can't Have Cake"?
Sheldon: It was obviously very effective. Leonard grew up to become an experimental physicist. Perhaps if she also denied him Christmas, he'd be a little better at it.