This study aims to better understand how acute pain and responses to pain treatment can be measured in children and adolescents using a non-invasive device. Pain is usually assessed using self-reported or observational scales, which can be difficult to interpret, especially in pediatric patients. This study will evaluate whether a novel device, the AlgometRx Nociometer, can provide an objective measure of nociceptive pain. Participants ages 6 to 21 years who are undergoing elective surgery or bone marrow transplantation will be enrolled at Children's National Hospital. The device measures changes in pupil size in response to gentle electrical stimulation, which reflects activity in pain-related nerve pathways. Measurements will be taken before and after procedures and during routine hospital care, alongside standard pain assessments. This is an observational study. Participation will not change or guide any medical treatment, and all pain management will follow standard clinical care. The information collected may help improve future pain assessment and treatment for pediatric patients.