FACS stands for Fluorescence-Activated Cell Sorting, which is a technique used in flow cytometry to physically separate cells based on their fluorescent properties. During FACS, the cells are first stained with fluorescent dyes or antibodies that bind to specific molecules on the cell surface. The stained cells are then passed through a flow cell, where they are illuminated by a laser beam. As the cells pass through the laser, their fluorescence is detected and analyzed by a computer in real-time. Based on the fluorescence properties, the cells are sorted and separated into different collection tubes or plates. FACS is a valuable tool in many areas of research, including immunology, stem cell research, and cancer research, as it allows for the isolation of specific cell populations for further study or analysis.