Well, I’ll tell you!
It’s the wide usage of the flush toilet. Although flushing toilets date back to 2300 BC. It was Sir John Harrington in 1596 who came up with the first practiced flush toilet. The wide spread use of the flush toilet came about in the early 20th Century as cities developed sewer systems and country folk developed septic tanks and leach beds. The expanded use of the flush toilet slowed the wide spread diseases that accompanied unsanitary conditions when disposing of human waste.
There are approximately 350 million flush toilets in America. Even with this many flush toilets, it is estimated that at least 1 million households don’t have flush toilets and are still using out-houses.
What a wonderful invention — sit down, relax, finish your business, flush, and everything disappears as if by magic!