Kid's mom: Becuase his dick is the size of a shrimp and he has to compensate by driving a SPT or small penis truck.
Kid: How do you know he has a small dick?
The Small Penis Truck gets its name from the owners of said vehicles. This type of truck is almost always owned by men who are compensating for the small size of their own genitalia. These men have a complex often referred to as Penis Envy. Because there is nothing they can do in order to increase the size of their own genitals, they attempt to show their manhood in other ways. Owning a giant, jacked up truck somehow, in their own minds, makes up for their like of penis size.
These owners of Small Penis Trucks are often dangerous as many of them feel that that they must constantly defend their manhood in violent ways. They are looking for ways to be manly. This can result in road rage, insulting others, starting fights and claiming that women that ignore them "must be lesbians".
This condition can only be cured if these men are mocked mercilessly. To help them, you should point and laugh at their trucks. Hopefully this will get through to them that the truck doesn't make up for their penis inferiority complex.