This is a good question. The rising action is quite beautiful. In this short story by O. Henry, there is a young couple in love and it is Christmas. Because they are poor they do not have money to get each other something. The story starts with these words:
"One dollar and eighty-seven cents. That was all. And sixty cents of it was in pennies."
The rising action is when Della and Jim, the two protagonists, decide to sell their one most prized possession to get each other a gift. Della cuts her hair and sells it. Jim sells his pocket watch.
After this, they are able to buy something for the one they love. Della buys a chain for Jim's pocket watch, and Jim buys combs for her beautiful hair. In the end, they have gifts that they cannot use, but they have at least shown love, which is the point of the story and why they are considered wise.