The act of surprise bombing your girlfriend.
Step 1: Wait outside the bathroom while your girlfriend is showering and
masturbate.
Step 2: As
soon as she opens the door, scream "THE JAPANESE ARE COMING!" and
splooge all over her leg.
Step 3: Repeat as necessary, because she'll probably go back into the shower to wash it off.
Alternatively, you could do a bombing run of sorts and jizz on her while she's still in the shower, as long as you include the scream.