In The Big Bang Theory (2007), s4e18, The Prestidigitation Approximation (2011), Sheldon (Jim Parsons) makes reference to when he was a child and got into trouble with Homeland Security after purchasing uranium to use for generating electricity. We see what happened in this episode.
First appearance of Tam, Sheldon's childhood best friend.
This episode was viewed by 12.66 million people and got a rating of 2.2 (adults 18-49).
In this episode, Mary (Zoe Perry) worries that Sheldon (Iain Armitage) is sitting alone in the cafeteria. Older Sheldon (Jim Parsons) explains that he actually liked being alone, pondering the mysteries of the universe.
Throughout the series Tam is often regarded as a brainy student like Sheldon, and at other times as just average. He meets Sheldon in the library over a book on rocketry, yet his own parents tell Mary that having a smart son like Sheldon is a great joy; a joy which they admit they don't have.