Answers to last issue's Do You Know?
1. I find that breathing through one nostril is easier for me than through the other. Is this a general phenomenon?
Answer: There is a counter-question to you: have you repeated your experiment at different times to check this?
The reason is this: apparently 85% of human beings only breathe out of one nostril at a time. Interestingly, they do not always prefer the same one, but switch to the other roughly every four hours. This switching frequency can vary from person to person, can vary depending on body posture or body position, nasal congestion, and even atmospheric factors like air pollution. 
A German scientist called Richard Kayser studied this "nasal cycle" as early as 1895, and found that it is due to an erectile tissue in the nose. It swells up in one nostril, mostly blocking it, and at the same time, the corresponding tissue in the other nostril shrinks, opening it up for breathing.
The two sides are not symmetric either. Scientists found that one uses more oxygen when breathing with the right nostril. Apparently, breathing through the right nostril may significantly increase blood glucose levels, while breathing through the left nostril can have the opposite effect. This has turned out to be a factor in certain cases of diabetes. On the other hand, the right hemisphere of the brain is more active when breathing through the left nostril, and the other way about.
Body posture alters the nasal cycle. If you lie down on your left side, the erectile tissue in the left nostril will swell up in about 15 minutes, and you will start breathing through the right. In fact, this is thought to be the reason why people sleeping on their sides switch sides throughout the night even when they are comfortable. But since one position lasts for about 4 hours, one or two switches would be sufficient.

2. How do atoms "know" which other atoms to bond with?
Answer: Atoms, of course, do not `know' anything like we humans do. They are not conscious beings. The question is better phrased as: "What determines the bonding of an atom with another?" The answer is: its chemistry.
Atoms form chemical bonds with other atoms when there is an electrostatic attraction between them. This attraction results from the properties and characteristics of the atoms' outermost electrons, which are known as valence electrons. They lie in so-called ‘shells’ around the positively charged nucleus. Each shell becomes stable once it contains a certain number of electrons.
When two or more atoms chemically bond together, they form a molecule. Sometimes the atoms are all from the same element. For example, when three oxygen atoms bond together, they form a molecule of Ozone.
When a molecule forms from atoms of different elements, we call it a compound. We are all aware of the chemical bonding of two atoms of hydrogen with one atom of oxygen to form a molecule of water.
Atoms bond with each other in order to make their arrangement of negatively-charged electrons more stable. Bonding allows atoms to achieve this stability by exchanging or sharing electrons with other atoms until each has their shells filled.
This is what happens when sodium and chlorine atoms bond because the outer shell of sodium can become stable by losing an electron, while chloring can become stable by gaining an electron. The two atoms are held together because losing an electron makes the sodium atom positively charged, while gaining an electron makes the chlorine atom negatively charged, and opposite charges attract.
Such an exchange is called "ionic bonding". Another way of bonding is to share electrons and this is called "covalent bonding". The bonds between the two hydrogen atoms and the oxygen atom in a molecule of water are covalent bonds.

3. I always get stomach aches before exams, can mental states like anxiety affect physical health so much?
Answer: Yes indeed, these effects are real, and such physical symptoms caused (or made worse) by mental states are called psycho-somatic. What are the physical processes that make this happen?
They are not bad, being in fact the processes meant to keep us safe. As animals, we have evolved to respond to danger, to either "fight" or to "flee". When we sense danger, our bodies get ready by creating two stress hormones: adrenaline and cortisol. This increases our heart rate and blood pressure, suppresses the digestive system, and affects the immune system. All this helps us use a great deal of energy to fight or run away. After the threat recedes, we experience a draining of energy, as our bodies return to rest.
Thus a certain level of anxiety is good, as it raises one's motivation. But if you are in a constant state of stress or anxiety, what doctors refer to as "chronic anxiety", your cortisol and adrenaline levels will constantly be high. This means that you return to a resting state less often, which can badly affect your body.
What is worse is that anxiety and stress may actually lower your pain tolerance: that is, make it worse for you to bear pain. The parts of the brain responsible for pain reception also relate to anxiety, and the two neurotransmitters (serotonin and norepinephrine) that are responsible for pain signaling in the brain and nervous system are also involved in anxiety and depression.
How do we handle the problem? One method is to use up that cortisol or adrenaline for good. If you can engage in some cardio exercise, such as a long walk, a run, or a dance session, it might help you take your mind off the stress, even if only temporarily.
Another way to deal with stress is to do some thing that calms you down. For some people this could be music, for some this may be exercise, or deep breathing, etc. Even a feeling of temporary relaxation can be good for you.

4. Why are gases invisible?
Answer: Actually, it is not true that gases are invisible. In fact, many are quite vivid and brightly coloured. Iodine vapour is a vivid purple. Chlorine has a yellowish green tinge. Nitrogen dioxide is brown-orange. 
Whether we see gases or not is actually because of how our eyes absorb light, or electromagnetic radiation. Light comes in various frequencies and we can see only a small range of frequencies. To understand the visible range, consider the rainbow as the range of light frequencies we can see.
For us to be able to see an object, it needs to reflect light in this visible range. Gas molecules, which are too small to see, do not react with light in the visible range. (And why do they not react that way? That is more complicated, and has to do with how electrons interact with photons.)
There are other gases in the atmosphere (particularly oxygen, carbon dioxide and water vapour) which also absorb light, but at ultra-violet and infra-red wavelengths that we cannot see, as they are either above or below the visible range. There is band of frequencies between the absorption spectra of oxygen and water where not much light gets absorbed. That is exactly the range of light that we have evolved to see.
Thus, it is not true that gases are invisible. It is just that we cannot see atmospheric gases, because they do not have colour in the visible range.
Sources: ScienceFocus, Scientific American, Healthline