another why question?

[cosworth34677]

why do trailer cost so much? i was trying to explain this to my girlfriend this morning and all i could come up with is they are made of metal. she quickly just
said well your trucks are mostly metal and they do more? so why does the thing
it pulls cost more the the truck? i had no good answer, so after mom look she walks away! i got to win this one guys,i hate when she makes since! (just kidding i love her)