When fucking a girl from behind and you spread her ass cheeks as she gets more excited and her anus expands-you spit into the hole without her noticing(the perfect example will not hit the sides). When she goes to the toilet post coitus (as women so often do, prevents thrush I am told) and the spit pops back out of her ass into the bowl with a splash.
Woman(from toilet): 'What the fuck?!?'
Man(from bedroom): 'Ha! I Eskimo Dropped you bitch!'