This study is the user testing phase of a mobile application intervention which will be designed to promote positive coping skills, illness resilience, and social support, and to serve as a complement to patient-physician interactions in adolescents and young adults (AYA) with sarcoma. User testing will be conducted with a prototype of the app. Participants will be AYA ages 13-25 with sarcoma (N=16). Study participation involves the AYAs downloading and using the app for 30 days. During the 30 days, participants will interact with the app by inputting their current mood, receiving supportive feedback from the app, reading messages, and doing brief activities (such as relaxation activities, etc.). Participants will also participate in an online focus group focused around the user experience. Before and after the 30 days of user testing, participants will complete a few brief questionnaires about their behavior, how their symptoms affect their life, and how they receive support from others. These questions will take around 20 minutes to complete at each administration.

Inclusion Criteria:

1. Parent or self-reported (for participants 18+ years old) physician diagnosis of sarcoma
2. Daily use of a smartphone (iOS, Android, or Windows) or tablet
3. Current Facebook account that participant can use to participate in the focus group (or, be willing to create a Facebook account for this purpose).
4. Able and willing to give informed assent and comply with study requirements
5. For patients aged 13-17 years, have one caregiver willing and able to participate in the study
6. Able to read and speak English

Exclusion Criteria:

1. Patients that are not able to participate in the study due to their medical condition and/or treatment regimen, as determined by the patients' treating oncologist in conjunction with patients' family members
2. Inability to provide informed assent/permission/consent
3. Inability to read, speak, and understand English