When 1 is loaded with a weight (1.168 mg of Al) and sandwiched between a high-temperature (34-37 ℃) heat source and a low-temperature (~0 ℃) heat source, a thermal engine that can extract kinetic energy from thermal energy is constructed. The crystal is ~5.2 mm long and ~43 μm thick (Figure ). As shown in Figure , the weight attached to the crystal moves with a large oscillation at a frequency of 6.84 (7) Hz. When the Peltier cooler is switched on, the crystal begins to vibrate, and the vibrations are gradually amplified until a large oscillating motion is achieved (Movie 3). This movement continues as long as the temperatures of the heat sources are maintained, and was confirmed to last for at least 160 h, corresponding to over 3.9 million cycles of bending and stretching. Unlike a previous example of molecular crystals that could extract kinetic energy from heat, 10 our crystal does not require periodic changes in temperature for actuation. Thus, to the best of our knowledge, our molecular crystal-based heat engine is the first of its kind. Figures and summarise the time dependence of the x and y coordinates as well as the velocity (v) of the weight, clearly demonstrating the high reversibility of the lifting and descending cycles (Movie 4). The average time interval for the descending and lifting processes over 70 cycles was 0.146(1) s. The maximum v (vmax) of the weight was 174(9) mm s -1 for descending and 179( ) mm s -1 for lifting (Figure ); these values are 5-6 times faster than the reported v of a glass bead (0.15 mg) pushed by a thermoelastic organic crystal (29 mm s -1 ). Figure summarises the thermodynamic cycle of the heat engine. As the crystal approaches the low-temperature heat source (273 K; state A), it deforms from its equilibrium position by Δq = ~5 mm (state B and Figure ) owing to softening. Using an elastic constant k of 2.94 mN m -1 (as summarised in Figure ), we determined the elastic energy (EEl) to be k(Δq) 2 /2 = 37 nJ. k rapidly increased to 5.48 mN m -1 when 1 was heated by the high-temperature heat source (298 K; state C), and EEl increased to 69 nJ. In other words, 32 nJ of EEl was supplied from the thermal energy. Based on the v and y (height) of the weight, the time dependence of its kinetic energy (EK) and potential energy owing to gravity (EG) can be calculated as mv 2 /2 and mgy, respectively, where g is the gravitational acceleration and m is the mass of the weight. Figure summarises EEl, EG, EK, and their sum (ET). The maximum EEl (~69 nJ) is greater than the maximum EG (~45 nJ), indicating that the EEl stored via the temperature change is sufficient to lift the weight to its highest position (y = ~3.8 mm). In other words, the EEl supplied from the heat source is sufficiently large for the fast and continuous motion of the weight. Based on the thermal energy absorbed by 1 as the temperature is increased from 273 to 298 K (1.0 mJ, as summarised in the caption of Figure ) and the supplied EEl (32 nJ), the energy conversion efficiency of our system was estimated to be 0.003%, which is similar to that of a photothermally driven crystal actuator. This heatengine behaviour is highly reproducible, and large vibrations could be observed even in crystals of low quality (bent or chipped) (Figures S25-S27 and Movies 5-8).