A morning erection that is naturally caused as the brains enters the REM-deep sleep phase. At this stage, the body'sskeletal muscle structure relaxes and causes hypervasodilation in the capillaries of the body, resulting in said erection.
After sleeping so soundly and having a great night's rest, it is no surprise to wake up with an erection.
Two people stand butt to butt and fart simultaneously. Performing this on the moon while naked can result in forces capable of granting the human body an escape velocity from the lunar surface.