Carrying a cat, they just feel so fluid, compared to a dog. Plus there’s that whole leaping and climbing thing.
Why is that? is there something different about their bone/tendons/ligaments?
Cat Anatomy - some extra thoracic vertebra and free-floating clavicles probably account for most of the flexibility. The rest is musculature and physical behavior.
If you are going to compare cats and dogs then we might as well talk of the differences between apples and oranges. It’s not like it’s six of one or half dozen of the other. 
Picking up a cat is like picking up a bag of rice, while picking up a dog is like picking up a sack of potatoes.
Because cats hunt by sneaking up on prey and then attacking in a burst of speed and power. Dogs are more long-haul hunters; they wear down their prey over time.
The loose-jointed anatomy of cats allows them distort their bodies to get extremely long bounds for great acceleration and high speed. But it’s a very energy-intensive way of moving - all muscle, with lots of up and down movement.
The dog’s tighter wound body allows them to move on more of a level and use the springiness of their tendons to store energy. They are less powerful but have far more endurance.
As such:
But both are great boiled, with a little salt, pepper, and butter.
No Photoshop required: http://2.bp.blogspot.com/-V3ZgZDraliY/UBMr06QOKGI/AAAAAAAA8jQ/l3XqRkDBuYs/s1600/cat-stretching.jpg
<ALF>Why are cats so fluid? They’re already halfway to cat juice!</ALF>
When my cats stretch, they stick out their rear ends towards me. I read somewhere that this is a sign of affection.
That’s a good source. A bunch of drunken frat boys (1972) and your mom (1985) also claimed this.
I didn’t say I heard it. I said I read it. Here’s one source:Yahoo | Mail, Weather, Search, Politics, News, Finance, Sports & Videos
Actually, I read it in a Yahoo Group, Cats Health and Behavior
Snakes are pretty fluid.
Jellyfish are almost entirely fluid.
The answer, in short, is “yes.” Cats are designed to be flexible. Their backs contain more vertebrae, more loosely attached, than most mammals. Their collarbones are unattached to any other bone, giving them exceptional flexibility there. They are, basically, built that way.
Now that we’ve had some factual answers…
Cats are not subject to normal laws of physics, THAT’S why!
Are they not, like, 75% water?
Cats are not normal. Period. 