I hope you're ready to confront a deep fear: rats coming out of your toilet. It's a thing that does happen—not to me, yet—but it's one of those realities of life that is so gross and distressing that you may not want to know how it happens.
Having said that, in this short video, National Geographic shows the step-by-step process. Get ready to freak out and add latches to your toilet seats. (The real nightmare fuel begins about one minute in.)
For more on this, National Geographic has a companion article: Yes, Rats Can Swim Up Your Toilet. And It Gets Worse Than That. Ugh.
Also relevant and freaky: 5 Animals that Might Show Up in Your Toilet.