The main answer is through the blood (circulatory system) and through various different types of neurons and nerves (nervous system). Human blood isn't just one chemical, it's actually hundreds of chemicals all being pumped along different channels. Oxygen, Carbon Dioxide, Blood cells, Hormones, Glucose and more are all carried from one cell to another in the blood, sort of like a highway in your body down which many types of vehicle can pass.
But a lot of the signals between different parts of the body are done via electrical signals sent along nerves and neurons. Your skin (arguably the largest organ in the body) is wired into your brain via the nervous system and can send it information about pleasure, pain, temperature and texture, all of which your brain processes, sending its own signals either down the nervous system or by triggering various glands (little chemical factories) which pump chemicals into your blood telling your body how to respond.
And these are just two two main systems. Your organs can also "talk to each other" indirectly by changing different parts of the body around them. Your muscles can produce acid which gets carried to the liver (mainly by the blood) and the liver responds by digesting it into less damaging chemicals, almost as if the organs have sent a message to the liver telling it to process the acid. Your body is really a network of different systems all communicating with each other whether you're aware of it or not. They do it in your sleep, when you're awake, when you're thinking about it and sometimes by accident.
We've all experienced the sensation of finally learning to do something after hours of failed practice. Somehow the brain seems to distinguish between successful and unsuccessful attempts. Aabha's question is a good one but the answer is very simple: nobody has even the remotest clue! What we do know is that memories are stored in different places around the brain and that when you reinforce a thought pattern, physical movement etc. your brain often becomes quite good at repeating it. But the brain is still flexible enough to allow you to try the skill (mental or physical) in different ways.
What's also interesting is that it doesn't happen instantly. When you learn to juggle, for instance, you don't suddenly find there's a moment when it clicks and you can juggle perfectly. You tend to find (I did at least) that once you start doing the juggle successfully, it still takes a couple of weeks of practice before you stop making the old mistakes. The same can be said of learning a fact in school, often it takes reinforcement before we stop making the previous mistakes.
Although, some types of information do seem to be like flipping a switch. If someone has always thought Mars was the colour green, it takes 30 seconds for them to learn it's red and they never make the mistake again.
All we can really say is that the brain has the ability to repeat its' activities, vary them, self-analyse, self-observe and compare actions with an internal set of "success criteria". Sometimes it can take a long time for the brain to learn the correct pattern, sometimes it can be instantaneous. Sometimes the brain rewards you for a new way of thinking, sometimes it punishes you (like when you lose sleep over a new frightening statistic you've learnt).
As a teacher, I wish I knew the secret to helping people learn something. If I knew (or if anyone knew) what was going on inside the brain when we learn a new skill, fact, way of thinking...I'd have discovered the silver bullet of education. Sadly our knowledge of the brain is nowhere near good enough to answer the question even a little bit. All I can say is that your brain can somehow distinguish correct from incorrect IF both options are made explicit i.e. you only know you've got it wrong if you know what right looks like.
A mirror is obviously a surface which absorbs very little of the light hitting it and bounces it back in all directions. So, by any reasonable definition, a mirror is white. But it doesn't look the same as normal white objects obviously. The reason for this is that there are actually two ways a surface can reflect light: Specular reflection and scattered reflection.
In specular reflection, every beam of light is bounced back perfectly at the same angle it came in. If a beam of light hits a surface at 60 degrees, it bounces out at sixty degrees. This is sort of "perfect" reflection and this is what's happening when we look at a mirror. In scattered reflection, the surface of the object isn't as smooth, so the beams of light do get reflected back at their original angle, but they get scattered in lots of different directions because the surface is jagged. This means we get all the beams of light coming out, but they are mixing with each other, crossing over and the original "image" is lost. This is what a white object is doing.
So a mirror is, in one sense, a perfectly smooth white surface, while a white object is a rough or jagged white surface (compared to the mirror of course). Perhaps we might want to say that a mirror is "mirror coloured" to describe "white specular".
A percentage is another word for a fraction. A fraction of 100 specifically. So to express 0 as a percentage of 0 we're asking the question: what is 0 divided by 0? This is an interesting question because there are four possible ways to answer it. In one sense we are taking an even number and dividing it by itself, making the answer 1 (100%). In another sense we are taking a zeroth of something, making the answer 0 (0%). In another sense, to divide by zero has an undefined solution, it's a quirk of mathematics that doesn't actually mean anything. Like if I said the words: Three plums diefy. That sentence follows all the grammatical rules of the English language and makes internally consistent sense. But it doesn't actually relate to anything in reality. Likewise 0/0 follows all the accepted grammatical rules of mathematics, but relates to nothing, so the answer is indefinable.
There is another fourth answer which Scientists tend to use. We tend to think of division by zero as being equal to an infinite solution. To put it crudely: dividing by 0 = infinity. There are definite examples where this can be shown to be the case, any inversely squared distance law gives the answer of infinity when dividing by zero, so Scientists think of it like that. However, Scientists also treat this as evidence that our hypothesis is wrong. If you divide by zero and generate an infinity, you've got something which can't be described in terms of the actual world, since nothing is truly infinite.
The best answer to the question is really to say that the question is grammatically sound but semantically meaningless. It doesn't actually make sense as a question and is, as such, unanswerable.
Travelling into the future is completely possible due to the effects of special relativity. If you move through space very fast, time will slow down for you compared to the rest of the Universe. You will only have travelled for a few minutes, but everyone else might have aged by several decades. So travelling to the future is definitely a real thing and astronauts do it all the time (on a small scale). As for travelling to the past, then it gets a little bit fuzzy.
The best answer is really to say nobody knows yet. There are some experiments in quantum mechanics such as the delayed choice quantum eraser experiment which do give the impression (possible illusion) of events in the present affecting choices in the past but it's very hard to confirm that this really is what's happening. In quantum field theory the notion of CPT conversation shows that time-reversal is allowed mathematically, and in fact may be happening at the particle level all the time, but this is again only an interpretation of experimental data and what's going on may be something else entirely.
At the end of the movie Gravity, Sandra Bullock's character manages to pilot and crash a module into an ocean, surviving the impact and making it to the planet surface. Oli's original question is: which planet is she landing on? The answer is definitely the Earth, for two reasons.
Firstly, the rest of the film takes place in orbit around Earth and to crash onto another planet would be too enormous a distance to travel. The distance between the International Space Station and Earth, compared to the distance to the International Space Station and Mars (the nearest planet) is like the distance between your left eye and your right eye, compared to the distance between your left eye and Antarctica.
The second reason we can decide it's Earth is that there are lots of plants around. At the moment, Earth is the only known planet in our solar system to have plants. If she'd landed on Mars it would have looked like a rough, reddish brown desert. Not a fun place to be. So the answer is definitely: she's crash-landing on Earth.
In Science we use a set of units called the Standard International Units. The kilogram, the meter, the second, the ampere, the mole, the Kelvin and the candela. All of our other measurements are taken by combining and defining these units. But in quantum mechanics we use a set of units called the planck units. There's a planck distance, a planck time, even a planck energy, mass and so on. The planck units are based on actual physical features of the universe. The planck distance for instance is the smallest distance across which the laws of physics are known to definitely work (anything smaller and we don't know). So why not switch to these units, why keep the SI ones?
The answer is mainly historical. In 1875 a meeting of Scientists took place to agree on International units to use, and it was called "The Meter Convention". It was here that they decided on the meter as the unit of measurement, the kilogram as a unit of mass and so on. They got their units largely from measuring the Earth and doing calculations on it. There is, in fact, an original kilogram made of platinum sitting in a vault at the Pavillon de Breteuil, likewise an original meter. The current SI units were largely decided at a meetings between 1875 and 1960, but the General Conference of Weights and Measures still meets every few years to review things.
While the planck system of units is, in a sense, a more fundamental property of the universe, the reason the GCWM use the current system is because it must be used by all Scientists all over the world, not just quantum Scientists. Furthermore, the quantum system is a lot newer and less familiar to people.
For me, the only SI unit which actually bugs me is the unit of ampere to measure electrical current. Because an ampere is a coulomb per second, I personally would have made a coulomb the fundamental unit of charge and made ampere the derived unit. But there are more people in the world who deal in current than actually deal with the property of charge itself, so the ampere sticks. So the reason we use the scales we do is mainly out of international agreement and "what's best for everyone to communicate" rather than "what's the Universe fundamentally doing". I can see arguments for both ways of doing things.