Once a rigidbody is moving at less than acertain minimum linear or rotational speed, the physics engine will assume ithas come to a halt. When this happens, the object will not move again until itreceives a collision or force and so it will be set to “sleeping” mode. Thisoptimisation means that no processor time will be spent updating the rigidbodyuntil the next time it is “awoken” (ie, set in motion again). For mostpurposes, the sleeping and waking of rigidbodies happens transparently.However, an object might fail to wake up if a static collider (ie, one withouta rigidbody) is moved into it or away from it by modifying the transformposition. This might result, say, in the rigidbody object hanging in the airwhen the floor has been moved out from beneath it. In cases like this, theobject can be woken explicitly using the WakeUp function.