How falling cats twist mid-air without pushing off anything

HostBack in the late eighteen hundreds, some of the smartest scientists in France got into a very heated debate over a few grainy, high-speed photographs of a cat. These pictures showed the cat being dropped upside down and somehow landing on its feet, which, according to the physics books at the time, should've been impossible. It seems like such a common thing we just take for granted, but why was this such a big deal to the experts?

GuestIt was a huge deal because it seemed to break a fundamental rule called the conservation of angular momentum. That's a fancy way of saying that if something isn’t already spinning, it can't start spinning unless an outside force pushes it. If you’re a cat falling through the air, you have nothing to push off of. You’re not touching a wall or a floor. You’re just... there. So, by the laws of physics, if you start your fall with your back facing the ground, you should stay stuck in that exact position all the way down.

HostBut we know they don’t. They flip. Are you saying the cat is somehow finding a way to push against the air itself?

GuestNot exactly. Air doesn't provide enough grip for that. Instead, the cat is doing something much cleverer. It treats its body not as one solid object, but as two separate parts that can move on their own. Scientists look at this using a two-cylinder model. Think of the front half of the cat as one cylinder and the back half as another, connected in the middle by a very bendy spine. The secret is that the cat rotates these two halves in opposite directions at the same time.

HostWait, if the front goes one way and the back goes the other, doesn't the cat just end up looking like a wrung-out towel? How does that help it face the ground?

GuestIt sounds like it would just cancel itself out, right? But this is where it gets really cool. The cat uses something called its moment of inertia. This is basically a measure of how hard it's to get something to spin. You can change this just by moving your weight around. Think of a figure skater. When they pull their arms in tight, they spin really fast. When they stretch them out wide, they slow down. The cat does this with its legs.

HostSo it's playing with its own weight mid-fall to change how fast each half of its body moves?

GuestExactly. First, the cat tucks its front paws in really close to its chest. That makes the front half very easy to spin. At the same time, it stretches its back legs out as wide as they can go, which makes the back half very heavy and hard to turn. So, when the cat muscles its body to twist, the front half spins a lot, while the back half—because it’s braced and heavy—barely moves at all.

HostThat gets the head and the front paws facing the right way. But then the back half is still trailing behind, isn't it?

GuestRight, so then the cat just swaps. It stretches its front legs out wide to act like a brake and tucks its back legs in tight. Now, the back half is the one that’s easy to spin. It twists its back around to line up with the front, while the front half stays put because it’s now the heavy, resistant part. By doing this quick tuck and twist sequence, the cat basically walks its way around in the air until it’s right-side up, all while its total spin remains at zero.

HostIt’s incredible that they can do that in a split second. But surely not every animal could pull this off even if they knew the physics. There has to be something special about how a cat is actually built.

GuestOh, they have very specific hardware for this. For one thing, cats don’t really have a functional collarbone. That lack of a rigid bone means their front shoulders can move almost completely on their own, way more than ours can. Their spine is also incredibly flexible. It acts like a universal joint, which is a part that can bend and turn in any direction at the same time. This is what lets those two cylinders—the front and back of the cat—operate on totally different angles without the cat snapping in half.

HostSo they have the bendy spine and the trick with their legs, but how do they even know they’re upside down fast enough to start the move?

GuestThat comes from their inner ear. They have a special setup called the vestibular system, which is like a tiny, built-in biological gyroscope. It’s incredibly sensitive to gravity. The very instant the cat starts to fall, this system tells the brain exactly which way is up and which way is down. This happens so fast that the cat usually starts its twisting move before it has even fallen a single foot.

HostIt’s wild to think that while those scientists were arguing over photos, the cats had already figured out how to use their own bodies to navigate the laws of motion.

GuestThey really are tiny masters of physics, using that flexible spine to turn a simple fall into a perfectly timed dance.

HostThose grainy photos from the eighteen hundreds were just the first time we caught them using those built-in gyroscopes to pull off a move that looked like magic.