Cats can fall from great heights without sustaining any physical injuries, while dogs often either die or get hurt from the same height. Why is this? It turns out that when cats fall from the air, they can quickly turn over and land on all fours. Most other animals lack this ability, hence the legend of "cats having nine lives". In 1894, French scientist Marey first captured the process of a cat falling with a high-speed camera and proved that the cat only needs 1/8 of a second to turn in mid-air. The phenomenon of cats turning over in mid-air is quite complex to explain: there is no external torque when the cat is in the air. If the front half of the cat turns, the back half must turn in the opposite direction, which would twist the cat's body. In the 1960s, Soviet mechanic Lozansky proposed the "tail-twisting" theory, arguing that the cat rapidly twists its tail to make its body turn in the opposite direction. This theory seems plausible, but the cat's tail is very light while its body is heavy. The tail would have to spin thousands of times per minute to enable the cat to turn over in a very short time, which is impossible. Interestingly, in 1960, British physiologist McDonald conducted a decisive experiment: he used several Manx cats born without tails as subjects, and the results showed that the cats' turning movements in mid-air were still agile and effortless.
There is also the "bent spine" theory, which argues that the cat first bends its spine from the middle during the fall, then bends it in all directions in turn, eventually making the front half of the cat rotate 180°. At this point, the back half of the cat must generate an equal and opposite moment of momentum to "counteract" the rotation of the front half, thus enabling the cat's entire body to turn over. In 1969, American mechanic Professor Kane used the mathematical model and computer program of the "bent spine" theory to conduct calculations, which perfectly matched the process captured by high-speed cameras. Currently, the "bent spine" theory is widely accepted. You can personally experience why bending the spine affects rotation: first, stand upright and keep your lower body still. You will find that rotating your upper body by about 90° will twist your lower body. The second time you bend over (that is, bend your spine), allow your upper body to have a certain angle with your lower body. At this point, your upper body can rotate many times around your lower body, just like playing a hula hoop. The characteristic of rotating during bending is that all parts of your upper body are making circular movements, but your face is basically always facing forward. Returning to the issue of cat turning over. When a cat falls, it bends its spine, and the axes of rotation of its front and back halves are not collinear. Therefore, the cat's front and back halves can rotate at any angle during the turning process without twisting. Cats bend over during the fall and use the opening and closing of their limbs and tails to perform aerial turns, which is a natural life-saving skill. Therefore, the legend of "cats having nine lives" really does have some truth to it.