A 6-year-old girl is referred to you by her pediatrician complaining of a pain in her throat and presenting with an anterior cervical midline mass for several weeks. After a course of antibiotics the inflammation and erythema resolved but the mass still persisted. The mother is concerned. She remembers a time when the child was 3-years-old complaining of similar symptoms, but then it spontaneously resolved. Physical chest exam is normal but the mass appears to rise upward when the girl sticks out her tongue. What is the most common diagnosis?