Samantha Weber knows what she's missing. She is 13. Her friends are starting to talk about boys, about school, to worry about the future.
"I'm supposed to be there," she says.
Instead, she is at St. Jude Children's Research Hospital in Memphis, Tenn.
A year ago, she learned the pain in her knee was cancer. She wears a brace that doctors hope will save her leg. She carries an intravenous feeding device in a bag on her shoulder, trying to regain some of the 20 pounds she has lost.
But she is smiling and sunny, even when she talks about the chemotherapy.
"The last chemotherapy I had, it just knocked me down," she says. "It was horrible."
Samantha just learned the cancer has spread to her lungs. She is realistic about what that means.
"It can kill me and I know that," she says. "This might be my last year or so here with my family. You just have to live it day by day."
Samantha's mother traveled with her to Memphis, but it is Samantha herself who is making the decisions with her doctors about her care.
"They tell me everything," she says. "They're not keeping anything from me, and I tell them what I think."
Samantha is not unusual. Researchers at St. Jude say even very young children can understand and participate in difficult end-of-life decisions.
Pamela Hinds, head of nursing research at St. Jude, was the first to ask the question, and she authored a study that appeared in the Sept. 19 online issue of Journal of Clinical Oncology.
Hinds says existing research in pediatric oncology focused on caregivers and on parents. She decided clinicians needed to understand more about the patients themselves.
Her study followed 20 terminally ill children between 10 and 20 years old to find out just how much they understood and how much they could participate in their own care.
"It's very common in health care for us to worry that a child would be too burdened by being involved in such serious decision making," she says. "We were stunned in this study to learn [that] not only are they quite able to participate, they're good at it. They know the options they have to consider, and they know the consequences."
The most striking finding, says Hinds, is that the main concern of the children was for others.
Children, "particularly adolescents, are thought to be quite narcissistic -- self-centered -- about their own desires," she says. "But our findings clearly counter that, and suggest that children and adolescents who have been ill for quite a while are making decisions in consideration of others, and at times at their own personal expense."
Samantha, for example, has agreed to try an experimental chemotherapy drug, even though she knows it will not cure her.
"They may not find a cure for me just yet with that," she says, "but they might find it for somebody else, save somebody else."
Jaleesa Williams, a 17-year-old patient at St. Jude, has made the same decision.
"I'm really glad to be able to help somebody," she says, even though the drug makes her sick. "Because if it was me, I would want somebody to do the same for me."
Lisa Anderson, the chaplain at St. Jude who knows Samantha and Jaleesa and countless other children like them, thinks she understands something of what drives their altruism.
"They have a very strong sense of finding meaning in this experience," she says, "and sometimes that meaning comes from being able to help others, in research especially."
In other words, children have as much desire to leave a legacy as the rest of us. These children are simply forced to learn that too early.
Correspondent Laura Marquez and producer Nils Kongshaug originally reported this story for "World News Tonight."