The bathroom sink has a slow drain.
I've filled it with liquid plumber, dumped pots of boiling water down it, the same.
You run the water to shave or brush your teeth, and up from the drain bubble yesterdays bits of blueberry smoothie, raspberry seeds, whiskers, gobs of undissolved toothpaste and shaving cream. You turn off the taps to let it all slowly percolate back down, a filthy ring forms about the sink, you attempt to rinse it, more bubbles up, you turn off the taps again...