Bones is offered a promotion to Private First Class (PFC/E3) as an incentive to continue doing his job well. Enlisted men in the US Army are not "promoted" until they reach the rank of Sergeant (SGT/E5). Changes in rank from Private (PVT/E1) through Specialist (SPC/E4) are treated as "advancements" because the rank does not convey additional responsibility, only additional pay and seniority. All advancements in rank from E1 to E4 are automatic after a certain amount of time in the Army. Promotion to SGT requires additional training (the Primary Leadership Development Course, at the time of filming) and selection by a promotion board. Bones, Jack, Christine, and Fred all graduated from water purification training at the same time, so presumably they have all been in the Army for the same amount of time. As a result, they would all advance to PFC at the same time. The one likely exception would be Fred, who mentions that he is a dental student. Dental school is a graduate level school, which means Fred already has a Bachelor's Degree. Anyone who enlists in the US Army with a Bachelor's Degree would start as a SPC (E4). Even if he hadn't completed his Bachelor's, Fred would probably have enough college credit to begin his enlistment as a PFC. Either way, Fred would likely be the senior ranking Soldier in the group. Finally, purified water in the Army must meet certain requirements, none of which include taste. US Army water purification units have neither requirement, nor the equipment to improve the taste of their water.
