One of my brother is a Importer, He Import goods from China. I asked he to carry me one day in the sea port. When he need to go in the sea port he said he will carry me there. Finally one day I went to the sea port, what I think about port is every country people has reason why they production in the factory and grow crops in the filed, those products people needs I mean they use them that is the reason they grow crops or production goods. To control port it could be airport or sea port they have some system to import goods from other country. just a example one country medicine could be other country poison. Lets say drug some country allow to have but some country not allow drug. Same way same product one country cheaper then other country. Basically we can say generation of nuclear bomb production, nuclear weapons production, medicine, drugs etc . I should stop my lecture, Really it was a wonderful day.