Once you have dry docked a toilet (see dry dock), you whip out your cock, and blow your load all over the mound of shit, thus capping the mountain with snow.
When a homosexual male defacates in the morning after homosexual intercourse. The residue from his partner is on top of his stool and is white in colour.
When Roger looked in the toilet after a crap he noticed a snowcapped turd, he then knew that Arthur hadn't worn a condom like he said.