Sheldon tells Penny that the last piece of food, the "greng jai (or kreng jai) piece," is reserved for the most deserving member of the group. While the concept of greng jai is difficult to translate into western-world languages, this is far from the truth. Greng jai is the custom of observing, acknowledging, and respecting the feelings of others - something that Sheldon is incapable of doing.
Sheldon refers to model railroad "gauges" (a term that refers to the spacing between the rails) instead of "scales". While most would see this as an unimportant distinction, Sheldon would never use incorrect terminology, and would hold anyone who did in contempt.