A rat’s ribs are hinged at the spine, enabling it to easily squeeze through the tightest spaces—like the pipes draining your toilet.
source/image(PrtSc): National Geographic
And rats are great swimmers too; they can hold their breath for up to three minutes.See how quickly a rat can go from the city streets to your bathroom.
The construction of toilets combined with a rat’s powerful survival instinct and ability to wriggle through tight spaces gives them a fighting chance to survive the perilous journey.National Geographic made a video showing how rats can move from sewers to toilet easily.
According to National Geographic, rats are talented swimmers, capable of holding their breath for up to three minutes and treading water for up to three days. Due to flexible vertebrae and ribs cages, rats are able to squeeze into tight spaces and even round the U-bend of a toilet./National Geographic