When you give a girl laxatives to cause her to have diarrhea, flip her over so her ass fills with shit (root beer), and cum on top of the pool of of shit (vanilla ice cream)
upon finishing the activity of an "upper decker" (shitting in the upper tank of someones toilet) you then dump an entire bottle of dishwashing soap in the tank and then flush the tiolet so the bowl fills up with bubbling shit water....
Where one splooges into someone's mouth and then proceeds to drop a sloppy deuce into their mouth. If the density of the poop is greater than the splooge, then the splooge will float to the top creating the illusion of a root beer float.