Why does burning stuff give out smoke?

During a fire, the air around it becomes heated. The heated air sweeps up water vapor (molecules of water that float in the air) and tiny specks of the fuel (the material being burned) into a dark cloud of smoke. If the temperature gets high enough, these compounds burst into flame. Once they start burning, there is no smoke because the hydrocarbons are turned into carbon dioxide and water (both invisible) when they burn.
 The more incompletely something burns, the more smoke it produces, because more particles are left to be swept up into the air. Smoke gradually spreads out and drifts away.

Why is honey the only food that does not go bad?

Pure honey does not go bad. Most honey is a supersaturated with sugar. Sugar being hygroscopic, its attractiveness to moisture makes honey a dehydrating agent. Most microorganisms do not grow in honey because of its low water activity. Molds and bacteria that land on the honey lose their own moisture(desiccate) to the honey, Thus their growth cycle is compromised and spoilage is reduced.
Over the time, honey may get crystallized. Crystallized honey can be re-liquefied by heating. You should never leave a jar of honey standing open. The supersaturated sugar solution will absorb moisture from the air and gradually become weaker, losing its anti-bacterial properties.

Why does mouth water when we see or smell delicious food?

When we see or smell good food, the brain registers that food is coming, and it instructs the salivary gland underneath our tongue to produce saliva in anticipation. Saliva helps break down the food as we chew it and makes it softer as it enters the gullet. As this is a subconscious reaction, it will occur even if we just see or smell the food.

Why do we get lumps in our throats when we hold back tears or are about to cry?

The ‘lump’ is actually not a lump at all but a counter-reaction to the body’s automatic nervous system. When humans are exposed to stressful situations,  the body automatically increases blood flow to vital organs and muscles. This is our typical ‘fight or flight’ biological reaction to things. Consequently, in order to get more oxygen to the body, one biological response is to expand the glottis in your throat.
But, being the social creatures we are, there are times when we try to suppress the urge to cry and often constrict or stop breathing altogether. Hence we have a battle between the body’s natural response to stress by widening the muscle in the throat to get more oxygen, and one’s wish to avoid shedding tears where we often stop breathing and thus constrict the muscle in our throat.  Hence, the vocal chords are fighting themselves to both compress and expand at the same time - and that's when we experience the tension as a lump. 

What does 20/20 vision mean?

During an eye test, eye doctors use eye charts to measure how well you see in the distance, compared with other human beings. The eye chart displays letters of progressively smaller size and it compares a person's vision to the that of the average for the population. "Normal" vision is 20/20. This means that the test subject sees the same line of letters at 20 feet that a normal person sees at 20 feet. 20/40 vision means that the test subject see as 20 feet what a normal person sees at 40 feet.
20/20 means that the person whose vision is being tested is normal.

What is blue moon?

A Blue Moon is commonly the name given to the second full moon in a calendar month. Since a full moon occurs every 29 1/2 days, if there is a full moon on the 1st or 2nd day of a month, there is a good chance there will be a second full or blue moon that month. Blue moons happen every two to three years. The moon looks like a regular full moon and not blue in color.
'Once is a blue moon' is frequently used expression which means ‘not very often,’ or ‘very rarely'.

Why do rain clouds float inspite of carrying water?

Rain clouds have bigger volume but very small mass,hence very low density (lower than that of air) because of which they float. Clouds generally form, survive and grow in air that is moving upward. Rising air expands as the pressure on it decreases, and that expansion into thinner, high-altitude air causes cooling. Enough cooling eventually makes water vapor condense, which under gravity falls as rain. So, even though typical clouds do contain a lot of water, this water is spread out for miles in the form of tiny water droplets or crystals, which are so small that the effect of gravity on them is negligible. 

Does a blood transfusion change our DNA?

Red blood cells, the primary component in transfusions, have no nucleus and no DNA. When blood is donated, it is processed in a number of ways but one important way is 'leukodepletion' - which is the removal of the white blood cells. If this wasn't done, the donated white blood cells would attack your body's tissues and cause inflammation.
A transfusion is supposed to give someone enough blood to last until they can make their own. So while it is theoretically possible to have other's  white blood cells DNA in a sample, those cells will eventually be eliminated from the body. 

What is the dot over letters 'i' and 'j' called?

Usually in many languages, specific characters are added to an alphabet to denote a particular accent.  This a mark added to a letter that is meant to signal a change in either the sound or meaning of a character.
The small distinguishing mark you see over a lowercase 'i' and a lowercase 'j' is called a tittle- which seems like a combination of tiny and little.  It was added to the letter i in the Middle Ages to distinguish the letter from adjacent vertical strokes. Tittle is also referred as a superscript dot or a jot.  

Why are umbrellas mostly black in color?

Basically, the umbrella was initially meant for rainy seasons. In the olden days umbrellas were made of thick cotton cloth. When they got wet in the rains, they would take a long time to dry up. It was found that umbrellas made of black cloth dried faster than those made of cloth of other colors. So it became a standard practice to use black cloth in the making of umbrellas. Black cloth dries faster because black or dark surfaces absorb more heat from the surroundings than lighter surfaces and as a result the evaporation process is faster.
Today, umbrellas are made of fabrics that do not soak water, and so, as drying is no longer a problem, umbrellas come in all shades and colors.

Why does blowing on a candle extinguish it?

To put out a fire you either need to remove the fuel, oxygen or heat. When you blow on a candle the movement of the air across the wick removes the heat from the flame. Cooler air takes its place. This lowers the temperature of the burning wax to below its ignition point. The wax stops burning and the candle goes out.

Why are there no B batteries, while A, C and D exist?

In 1924,the ANSI - American National Standards Institute, USA, proposed a simple system to designate cell sizes, based on the letters of the alphabet, starting with the smallest cell in volume and going in ascending order, A, B, C, etc.
B batteries never really caught on commercially, at least on the consumer end of things.  B batteries were primarily used in devices that used vacuum tubes which is outdated technology. A size batteries are hard to find as well, because they were primarily used in really old laptop batteries.
The ones that were most popular just ended up being the AA, AAA, C, and D. Most batteries (AA, AAA, C, D) all carry the same voltage, 1.5 volts, but they differ in their capacities, their physical size.

Are zebras white with black stripes or black with white stripes?

Zebras are native to central and southern Africa. They are black with white vertical stripes on the head, neck, forequarters, and main body, usually with horizontal stripes at the rear and on the legs of the animal.
Zebras are dark animals with white stripes where the pigmentation is inhibited. The pigment of the hair is found solely in the hair and not in the skin.

How far can the human eye see?

You see because the light reaches your retina. You can see as far as a light source can reach your retina with enough intensity  How far the human eye can see depends on how many particles of light, or photons, a distant object emits. Theoretically, there is no limit to the distance we can see, the object just has to be large enough or emitting enough radiation. So the human eye can see until its line-sight hits an obstacle. Healthy human sight can see an almost unlimited distance. Normally vision is stopped by something blocking it. If you are looking at the horizon the curve of the earth stops you from seeing farther. The farthest thing that a human can see without a telescope is the Andromeda Spiral galaxy, which is about 2 million lightyears away.

Why cant we drink sea-water?

Our bodies are mostly water (~75%). The water in our bodies supports many functions, including waste removal and temperature regulation. Since water is consumed and expelled by these functions, we need to constantly replenish our water reserves to survive. In normal circumstances, the saltiness of water in your cells is the same as the saltiness outside your cells. But when your cells consume water, the solution becomes more concentrated, and the natural process of osmosis allows water from outside your cells to pass into your cells to achieve equilibrium. That’s how we absorb water as per need.
The opposite happens when drinking sea (salt) water. The salinity outside your cells increases rapidly and water from cell moves out, causing the cells to dehydrate. A person may die of extreme dehydration.

Why does Earth rotate from west to east?

The angular momentum of pretty much the whole solar system is in the west to east direction. All the planets, the sun rotate that way. Venus and Uranus are the exception, as they rotate differently.  According to astronomers the rotation and orbits of the planets and stars are largely determined by conservation of angular momentum.  The direction of the rotation largely relies on gravity and other conditions in play as each celestial body is formed and enters its orbit.

Why do we have two eyes but see only one image?

When viewing with two eyes, the muscles that control our eyes work automatically to focus both eyes on the same spot. Each eye receives light photons that are focused upon the retina by the lens to form an image. The 'image' is transmitted by the optical nerves of both eyes to layers of brain tissues at the back of the head. The brain then processes both the images to form single 3D image. Basically, if you close one eye, you can still see, but in a single plane. So even though your brain tries to add depth, what we see is a flat a flat image with one eye. But with two eyes next to each other, you can you can view the relative depth.

Because the eyes are a few inches apart the images are slightly different permitting depth perception. The left and right eyes balance the image’s depths and shadows. This is the reason why we see the image shapes. Both eyes together help to see the distance of the object.

Why does a smile make you happy?

Smiling usually happens without much thought. It is one of the most important nonverbal communication skills we possess.  Smiling just using the muscles in the face to turn your cheeks upward releases endorphins that aid in creating a positive mood. Sometimes your joy is the source of your smile, but sometimes your smile can be the source of your joy. Scientists have found compelling evidence that smiling causes people to feel happy, regardless of how they felt at first. Smiles are infectious. So even if you don’t feel much happier, the people around you are more likely to smile, and that can improve your mood as well. Smiling can reflect inner happiness, but it is also a great way to give happiness to others. One reason smiling faces trigger increased happiness is that the set of the facial muscles a smile produces is associated with happiness by the brain.
Human being is  social animal and our well-being often depends on our ability to gauge the emotions of those around us. Smiling is one major social cue. Smiling is a socially accepted feelings transmitter.