It's expected to bring a gift to a birthday party.
John's classmate is having a birthday party and the whole class is invited.
John wants to go to the birthday party.
John neglects to get a present for his classmate and goes to the birthday party with nothing at all.
John buys a nice birthday present for his classmate and brings it to the birthday party.
John's classmate thinks John is weird and stingy for coming to his birthday and not bringing a present.
John's classmate loves John's present and thinks John is very generous.