Aliaksei Zholner built this incredibly small, single-cylinder engine entirely out of paper and it works! For those interested, Aliaksei shared his build process on the Russian forum, only-paper.ru

Specs below:

LxWxH: 18 x 13 x 22 mm
Shaft diameter: 0.5 mm
Piston stroke: 3 mm
Cylinder diameter: 3 mm
“Swept volume”: ~ 21 mm3