Rotors are used in many applications to exchange energy between a mechanical system (vehicle, turbine) and the surrounding fluid (such as air or water). This image shows the extreme deformations of a small rotor fitted with two flexible polyethylene blades when operating in water. Depending on the frequency of rotation, the speed of the incident flow and the inclination of the blades, largeamplitude deformations may appear in either direction, affecting its performance. In a liquid, this phenomenon can be very pronounced since the forces are proportional to the density of the fluid. Studying these surprising behaviours could lead to possible improvements for tidal turbines and underwater drones. This image is one of the winners of the 2022 La preuve par l’image (LPPI) photography competition.