Why did kissing evolve? If we want to find out, we’ve got to decide what counts as a kiss first.

What’s the weirdest thing you learned this week? Well, whatever it is, we promise you’ll have an even weirder answer if you listen to PopSci’s hit podcast. The Weirdest Thing I Learned This Week hits Spotify, YouTube, Apple, and everywhere else you listen to podcasts every-other Wednesday morning. It’s your new favorite source for the strangest science-adjacent facts, figures, and Wikipedia spirals the editors of Popular Science can muster. If you like the stories in this post, we guarantee you’ll love the show. By Rachel Feltman This year’s Ig Nobel Prize in biomechanics went to researchers trying to answer a surprisingly complicated question: what’s a kiss? Their definition—”non-agonistic interactions involving directed, intraspecific, oral-oral contact with some movement of the lips/mouthparts and no food transfer”—was concocted to exclude such exchanges as ants feeding each other with their mouths, bees and birds vomiting at one another, and fish that fight by smashing their open jaws together. Defining a kiss might seem like a silly scientific endeavor, but it was a crucial first step for their research. Once they had definite criteria for a kiss in place, they went looking for that specific behavior in research about all sorts of primates. By mapping the appearance of kissing the same way they might map any other evolutionary adaptation, they came up with an estimation of how far back the trait could have evolved. The number they landed on is some 20+ million years. Listen to this week’s episode to hear more about the 2026 Ig Nobel winners, and to learn why kissing is such a mysterious—and unique—human behavior. FACT: The “American cheetah” wasn’t a cheetah at all Featuring Riley Black This week’s episode features special guest Riley Black. Her new book, Tyrant Lizard Queen: The Love, Life, and Terror of Earth’s Greatest Carnivore, comes out later this month. For her guest spot on this week’s Weirdest Thing, Riley decided to tell us all about the fastest—and perhaps fattest—cat in North America. For decades, paleontologists thought that an extinct cat called Miracinonyx evolved into a cheetah-like springer specifically to chase down North America’s speedy pronghorns. It was a tidy, romantic story of predator and prey evolving together in lockstep until one had gone extinct; it explained why pronghorns were so dang fast, too. But Riley always found the story a little fishy, and she wasn’t the only one: in 2023, researchers noted that these American cats had forearm bones more suited for grasping prey (cougar-style) than for running super fast (like a cheetah). New research has put another nail in the coffin of the “American cheetah.” For starters, their fossils were found as far north as the Yukon—where they wouldn’t have had any pronghorns to hunt. And based on their DNA, the researchers say, they’re much more closely related to modern pumas. They also had a unique diet: according to isotope analysis, the researchers think the Yukon-dwelling cats feasted on salmon. That leaves us with one big question: what were pronghorns running from? FACT: Rich people love building little stone houses for themselves (to be dead in) Featuring Amanda Reed Old friend-of-the-pod Amanda came back to join us this week. She took us on a tour of mausoleum history. Walk into Pittsburgh’s Allegheny Cemetery, and you’ll find it hard to miss the sphinxes. They’ve got ample bosoms, appropriative headdresses, and nothing whatsoever to do with actual Egyptian funeral traditions. They’re actually monuments to the fortune (and ego) of the late industrialist Emil Winter. Emil, who got to move into these fancy digs in 1941, was not unique in desiring such an opulent postmortem dwelling. In fact, he decided to have it built after seeing a similar one by the same architect: the Woolworth family mausoleum in New York City. Listen to this week’s episode to learn more about humanity’s enduring desire to build fancy houses for the deceased. 2026 Popular Science Home of the Future Awards 39 products that will improve your everyday life.