A femtosecond laser is used to perform "nano" surgery on a living cell. The actin filaments in the cell have been fluorescently labeled using green fluorescent protein (GFP). When a single filament is cut, it retracts, much like a violin string that has broken. This demonstrates that the actin filaments in the cell are under tension, and hence are being "pulled" apart. As a result, the filaments retract when they are cut, as shown in the figure. This provides critical insight into the behavior of cytoskeletal network that plays a key role in determining the mechanical properties of the cell.