Sex act involving either being or dressing up like a ninja. Begin by quietly dropping out of the darkness/ceiling or creeping into a darkened room to find your girlfriend fast asleep. Quickly and quietly achieve orgasm, blowing your hot load allover her unsuspecting face. Throw down a smoke bomb and make your escape out of the nearest window using a grappling hook or some other sort of urban climbing device. Loose the ninja suit, return to the scene of the crime, and act as if you have no idea whose semen is allover her but make sure and exclaim that it is by no means yours.
Isn't she just angelic when she's asleep like that? It's like she's just quietly begging for a hot ninja.