It's a trick to make the simulation more stable. Stacking in physics engines is difficult. It's a careful balance between speed and stability. If this demo too

It's a trick to make the simulation more stable. Stacking in physics engines is difficult. It's a careful balance between speed and stability. If this demo took place on Earth, the Jenga blocks would have to be 1.96 meters long in order for the gravity to be realistic. Alternatively, if the Jenga blocks were the standard length of 7.5 cm, this would take place on a planet where gravity is 0.375 m/s² (less than the Moon). If you want to see what the simulation looks like with Earth's gravity, change line 46 of examples/jenga.html (at commit e067679006a92dbbd02a81e990a69a86a5812e81) to scene.setGravity(new THREE.Vector3( 0, -784, 0 ));