Rats are up there with snakes and spiders when it comes scaring the pants off people. One glimpse of a beady-eyed, yellow-toothed rat scuttling across the basement floor or darting down a city sidewalk is enough to make most people scream. There is much to admire about rats (they are smart, surprisingly well-groomed, and they make excellent pets). But let’s face it: They are also the stuff of nightmares. These facts about the two kinds of rats that love to live around people—brown rats (a.k.a. Norway rats, Rattus norvegicus) and black rats (a.k.a. roof rats, R. rattus)—are drawn from the book Frightlopedia. They will fill you with terror, or awe, or perhaps both.

1 YOU CAN'T KEEP THEM OUT ...

A rat can squeeze through a hole the size of a quarter, thanks to its collapsible skeleton. Its ribs are hinged at the spine and can fold down like an umbrella, which means that any hole that’s big enough for a rat’s head is big enough for the rest of him.

2. ... NOT EVEN WITH A BRICK WALL.

Rats can chomp their way through thick wood, metal pipes, brick walls, and cement. Their front teeth are long—they grow about 5 inches every year—and also very sharp, with a nifty self-sharpening feature: The edges of the upper and lower teeth rub against each other, having the effect of a knife on a whetstone.

3. WHEN THEY BITE, THEY DON'T MESS AROUND.

Rats will usually only bite when cornered. But then they bite hard—very hard. Their jaws are built like an alligator’s and can exert as much as 7000 pounds per square inch—which means their teeth can easily slice down to human bone, as one biologist for New York State discovered when he picked up an errant lab rat with his hand. "It put its teeth straight through my index finger," Stephen C. Frantz told Richard Conniff in his book Rats! The Good, The Bad, and The Ugly. "The membrane over the bone is quite sensitive, and it was grinding its teeth back and forth. I get chills thinking about it."

4. THEY ARE SUPERB ATHLETES.

The long claws on a rat’s feet allow it to scale brick or cement walls with Spider-man-like ease. Getting down isn't a problem, either: A rat can fall 50 feet and land on its feet without injuries. Rats are also phenomenal jumpers; they can leap 2 feet in the air from a standing position. With a running start, rats add another foot to their leaps—which, according to Conniff, is equivalent to a person jumping on top of a garage. According to one study [PDF], rats can lift objects that weigh nearly a pound—more than the average rat’s body weight.

5. YES, THEY CAN SWIM UP YOUR TOILET.

Rats can swim for three days straight (in laboratory conditions), they can hold their breath underwater for up to three minutes, and they can perform their skeleton-collapsing trick while swimming. All of which means that, yes, they can paddle through sewer pipes, squeeze through your plumbing, and pop up in your toilet.

6. THEY'RE SEX MACHINES.

Rats leave rabbits in the dust when it comes to reproducing. “If they are not eating, rats are usually having sex,” writes Robert Sullivan in his delightful book Rats: Observations on the History and Habitat of the City’s Most Unwanted Inhabitants. During a single six-hour period of receptivity, a female rat may mate as many as 500 times, which helps explain how, according to Sullivan, a pair of rats can end up producing 15,000 descendants in one year and why they are the most common mammal in the world.

7. THEY COME IN SIZE XXL.

On average, the brown rat is about 16 inches long (including its tail) and weighs less than a pound. If a rat lives near a steady food source, like a dumpster, it can grow to be 20 inches long and weigh 2 pounds. But that’s tiny compared to the Bosavi woolly rat, which was discovered in 2009 by a BBC expedition to an extinct volcano in Papua New Guinea. The 32-inch-long beast weighed more than 3 pounds and showed no fear of humans. It’s thought to be one of the biggest in the world, and it’s a “true rat, the same kind you find in the city sewers,” mammalogist Kristofer Helgen told the BBC. Don’t worry about meeting it on the street, though: The rat, which is believed to belong to the genus Mallomys, lives only in the area of the volcano.

8. THEY MAY BE LAUGHING AT YOU.

Rats make high-pitched chirping sounds (especially when they're being tickled by humans) that humans can’t hear, but which scientists think may be the equivalent of laughter.

9. THEY HAVE VERY SENSITIVE TASTE BUDS.

It’s not easy to poison a rat. The animals can detect infinitesimal amounts of poison in food—as little as one part per million. "That’s like being able to taste a teaspoonful of chocolate in 1302 gallons of milk," Conniff writes. Rats are also cautious when eating unfamiliar foods; they'll start by eating just a tiny bit to make sure they don’t get sick.

10. RATS PREFER THAT YOU DO NOT WASH YOUR FACE.

Occasionally, where there are heavy infestations, rats will bite people’s faces and hands at night while they sleep, drawn by food residue on their skin. That might be a good time to move to a new town because, once a rat bites you, the rat’s chances of biting another human go way up. It’s like how, after finding a new favorite food, you order it at every restaurant you visit. In 1945, Curt Richter, a biologist at Johns Hopkins University, fed human blood to captured rats [PDF] and concluded that "a strong craving for blood might explain why, once having bitten a person, the rats apparently are apt to bite another."

11. RATS CAN SURVIVE NUCLEAR EXPLOSIONS.

During the 1950s, roof rats living on Enewetak Atoll in the South Pacific confounded scientists by surviving atomic bomb testing. While the details are sketchy, it’s believed that early nuclear testing obliterated the Polynesian rat population, and that the atoll was then repopulated with a different species of rat that burrowed deep down to survive future testing.