Bruce Lee said: be like water. Water fills the cup that it flows into. Likewise the teapot.
What he may not have said is the reason that water flows. It flows because gravity pulls it. Without gravity water would do nothing but float around. Gravity gives it direction. But it also needs a shape, a channel to guide the way it flows. To fill a cup water has to be poured into it. The spout of the teapot guides it.
But how does water actually fill the shape of a cup? No matter what the shape water can, for the most part, fill an open container.
Water is made up of lots of tiny little elements that can move and shift relative to each other. You could think of water as having a sort of flexibility. But this flexibility by itself wont' cause it to fill a cup. It needs some way of sensing the cup so that it can fill it. In a way it is gravity that provides this "sensing" ability. Gravity pulls it down and because of that pull water is forced to look for a path to flow along. This is both relative to itself (the particles of water to each other) and to the container. Water senses so that it can flow.
In order to flow water needs something to pull it. It also needs flexibility, the ability to mould into different shapes, and it needs sensitivity, the ability to sense the areas that it can flow into.
We could think of a yoga pose as a shape that we pour our body into. Rather than being pulled by an external force, we create the driving force internally, with intent. To flow into the shape of a pose, we then also need flexibility, which is provided by our joints, and sensitivity.
Water doesn't think when it flows into a cup. And even if it could think, it isn't exercising that facility when it is flowing. Instead it "senses" where it can go and it goes there. If the cup were to change shape as the water is poured, or if the cup is moved, then the water still continues to fill it, perhaps it sloshes around a bit, but even as the cup is changes shape or moves, water finds the way to fill it.
It doesn't think "dammit the cup has moved, it has changed shape! It just gets on with filling it, assuming that the cup is still somewhere between itself and the center of the earth.
Flowing then is an absence of thought. It involves having a clear intent, something to guide or pull us (a yoga pose or an action, or a desired effect) and it involves the use of the senses and the ability to change the shape of our body.
In kung fu, in any "skill" in general, we practice to make a skill a part of ourselves. We practice different blocks and strikes, or we practice different yoga poses, or we practice different ways to write a computer program. Initially we practice a set of skills in the same sequence so that we can get imbed those skills into ourselves. An example, the alphabet. We learn the letters in order so that we can remember them easily. But once learned we hardly even need the alphabet. We instead need the letters that make it up.
Breaking letters free from the order of the alphabet, and instead using the letters of the alphabet in the new context of words and sentences, we can communicate freely.
Learning any new skill, we initially learn them in a set order to learn them. But once learned, we can break the rules that we first learned them within so that we can play within new rule sets.
That's how we learn to flow.
Water flows into a cup, and fits the shape of that cup. But it doesn't remember the shape, or else how would it flow into new cups. What it remembers is how to fit into the shape of whatever it is flowing into.
Water doesn't always flow though. Sometimes it's difficult. If you've ever bled a brake system, or a radiator, you know that air bubbles block the flow of fluid. This could be equivalent to blockages in our ability to feel our body or control it. When we have blockages in our senses, we can't sense what is happening now. If the blocks are in our ability to respond, then even when we can sense a new cup, we can't flow into it.
And that may be the goal of practice, learning to remove the blocks from our senses and our responsiveness so that we can flow. And that's the advantage of an organized system. It helps us to see where the blocks are so that we can remove them. Learning the alphabet in order helps us to learn all 26 letters without missing anyway. Likewise learning an organized system of kung fu or yoga.
But then once we have learned the essentials, it's time to let go of the initial form that helped us learn those essentials so that we can be like water and flow.
What are the essentials of flow?