Generally this occurs when you have someone of high precedence drooling lazily on your lap during the popcorn moon at gasmark 5. They will feel left out and violated as their drool notices it's been caught on the rebound from the previous not-quite-so-well-washed-out drool related crotch stain. The drool will become angry, feminine, masculine and a hard unreactive solute.
Under the clever rouse of slipping into something more comfortable the new-comer drool will proceed to put together The Ultimate Shotgun noisily in the hallway casting shadows into your vision that subtley give the game away. Well that and the drool's asking your assistance in putting together the ultimate shotgun and it's repeated shouting of it's intention to Shoot your longest armpit hair with it. When it's least expecting it walk very slowly away and pray for f*ck that you won't experience a bullety, messy exploding sensation.
Barry Scott had a crotch stain.Edit
BEWARE!!! Virtual crotch stains! (sharp inhalation of stool-ridden air in shock!) By sinisterly hiding inside a porn video on an internet pop-up you will be legally obliged to either where a virtual reality headset forcing you to see an ominous stain in the round-about location of your wang, or force you to print out a picture of a crotch stain as designed by a lucky BluePeter contest winner and stick it onto your trousers around your coin-purse.