Senior Airman David McMillen, 28th Civil Engineer Squadron heating and air conditioning technician, applies caulk paste to sheet metal to create a seal on the side of dock 21 at Ellsworth Air Force Base, S.D., Feb. 2, 2012. By properly installing the system, technicians ensure efficient use and conservation of valuable resources in the facility. (U.S. Air Force photo by Airman Alystria Maurer/Released)