An animal is a society of tiny single celled organisms, that is growing by way of these living cells giving birth to more and more cells and informing each other and rearranging each other.
To what kind of human venture is it most like? The founding of a city? So pretend one person washes up on a beach a lush tropical island beach and for the hell of it, pretend people are unisex and reproduce by splitting in half. (quite odd, eh?) so this person eats all the cocounts and mangos and gets fat and splits. At first the fellow is doing everything, finding food, chasing away monkeys, building fire, foraging for supplies, building shelter... then the twins split and they split...
Now at first they are identical twins. But soon the colonists begin specializing in different tasks, and the more each twin specializes and perfects his craft he interacts differently with his other twins, and forces them to change their habits too. now, all the while they are still splitting so guilds form and specialization within guilds happens. A village starts to form, with twins building huts around themselves and some roads to make it easy to move resources and materials around. And then some guilds start specializing on building and roadmaking... Since they are all clones of each other, they form a very closeknit and well coordinated society.
As the village grows, more specialization happens, different groups start forcing each other to reorganize, rerouting roads etc.. More structure forms, more organization, more complex interactions, materials are processed in more elaborate chains of production. As storms come, seasons change, land gets used up, visitors come, possible invaders... the village begins behaving in more complex ways, forms complex governments, diplomatic missions, armies, traders, politicians...
In essence, the city is built from within by its 'cells'. That's how animals form. Now to make our city more like an animal, imagine the city structure build like a giant robot like one of those transformer toys. Instead of being built on the ground, the whole structure is being built in 3 dimensions, the whole city structure is flexible capable of walking about, or even flying. A giant honeybee robot!
To get closer to being like an organism, lets add one more layer of complexity. Most of the scaffolding and mechanisms of an animal are made of the cells themselves, (with some nonliving bone and fiber built in for strength...). Now, a honeybee is a million cells. That's how many people in a fairly big human city, say just manhattan. The reason that the organs and tissues of an animal are so much more flexible and responsive than human city structures is that the animal organs and tissues are build out of the living cells themselves. So all the structures are dynamic, responsive to each other.
So imagine the roads are lined with these ... ok, so imagine these colonists as not QUITE humans, a little dumber, a little more able to settle for performing very simple roles. As indeed some animals are, like oysters, and various simplified parasites... which start off as flexible swimming sensing larvae and then settle down to become basically, sessile, eating tubes producing eggs.
So imagine these colonists, making buildings out of themselsves, holding onto each other. Imagine the elevators are groups of people some holding each other making a basket for other people to ride in while the others do the climbing up and down. Some are the windows opening and closing because the 'people' that form them can sense the environment and respond the the people inside. The scaffolding is people holding onto each other so it can respond to changing loads , the building can shift over if it needs to. Cars and busses are simply bunches of people holding onto each other walking with their legs letting riders sit on them, some of them use their eyes to see, steer, communicate with other cars, some collect food and distribute it to the other 'people' that make up the car.
So all the structures and machines in the city are alive, are made of 'people' that can communicate with each other, allowing the structures to respond to each other and even think how to respond.
So that's what animals are. Cities made mostly out of societies of simpler single celled organisms. You and I are stupendous cities made of a trillion cells. that's like one hundred thousand new york cities full of people. One step in realizing that life is not a shear magical mystery is realizing the numbers involved, the complexity capable of those numbers.
If you wonder how a brain can think, realize that it is made of a hundred billion neurons. A society of one hundred billion simple critters capable of making simple decisions, inventing tiny snippets of ideas, memorizing a word or two at a time, all having conversations with each other. Typical neurons are connected to a thousand others at a time, and synthesizes information from all of them while then sending out signals to hundreds of others in the network. This society takes over 30 generations to form (typically a cell takes about a day to divide into two cells) Over the time of these 30 generations the cells form all kinds of social bonds and rearrangements and habits of communication...
And, if these cells that build the organism-city out of themselves, are themselves organisms too, simpler ones, what are THEY made of? If they are like animals, do THEY have organs and parts? Are THEY cities of even SIMPLER sub-cellular organisms?
Kind of...This living world is made of quite a few levels nested in deeper and deeper... Very strange universe! In the next chapter we will explore the world of those subcellular parts - molecules, what are they and What can they do?