(a) Side view of a rotary engine. Wet paper provides the humidity gradient. Blue plastic blocks weighing 15 mg attached to HYDRAs increase the amount of mass shifting position relative to the axis of rotation. (b) Photos and (c) measurements showing the horizontal shifts in the positions of plastic blocks attached to HYDRAs. Markers indicate the average data values with error bars showing the s.d. calculated from measurements on five samples. (d) Rotation speed measured as a function of external relative humidity and at two different airflow speeds near the device. The rotary engine can drive a vehicle forward if its rotation is coupled to the wheels. (e) Snapshots showing the position of a miniature car driven by a rotary engine (see Supplementary Movie 5). Scale bars, 2 cm (a,e); 5 mm (b).