Dr. Leonard Hofstadter, a physicist at Caltech, got his PhD at the age of 24 thanks to his 173 IQ. Leonard lives with his best friend and co-worker Sheldon.
Leonard is definitely the most social of his nerd group of friends and has a crush on their neighbor, Penny. Leonard went on a date wih Penny at the end of season one, but it didn't work out due to their insecurities.
During the third season of Big Bang, Leonard returned from his artic expedition with the guys and began dating Penny.
Penny: What actor holds the record for being named People magazine's Sexiest Man Alive?
Sheldon: William Shatner!
Leonard: I don't think it's Shatner.
Sheldon: Then it's got to be Patrick Stewart
Sheldon: Here's the problem with teleportation.
Leonard: Lay it on me.
Sheldon: Assuming a device could be invented, which would identify the quantum state of matter of an individual in one location and transmit that pattern to a distant location for reassembly. You would not have actually transported the individual, you would have destroyed him in one location and recreated him in another.
Leonard: How about that.
Sheldon: Personally, I would never use a transporter because the original Sheldon would have to be dissintegrated in order to create a new Sheldon.
Leonard: Would the new Sheldon be in any way an improvement on the old Sheldon?
Sheldon: No, he would be exactly the same.
Leonard: That is a problem