Today we launch the FQxI "My Favourite" short series, where our members discuss their favourite equation or paper. In the inaugural episode, Nicole Yunger Halpern talks Carnot's theorem, highlighting its significance in thermodynamics. Carnot's theorem is a fundamental principle in thermodynamics that reveals the maximum theoretical efficiency of heat engines. Developed by French physicist Sadi Carnot in 1824, it demonstrates that no heat engine can be more efficient than a reversible engine operating between the same temperature extremes. Carnot's theorem laid the groundwork for understanding energy conversion and the fundamental limits of mechanical work. Stay tuned for the next episode, where another FQxI member reveals their most cherished scientific inspiration.