Why is it easier to tear paper along a crease?

Paper is made from wood, and wood is made from long fibers. Paper is made out of cellulose fiber material, and these fibers line up in a particular direction, during the manufacturing process.  There is a distribution of the fibre sizes,  the finer ones penetrate into the web of larger ones holding them together. Typically the manufacturing process leaves the fibers are more or less parallel. So it is easier to tear in the direction that separates fibers from neighboring fibers than in the direction that breaks fibers. Creasing paper breaks and/or separates some of the fibers, making it easier to break/separate the rest. Excessive weakening of the paper happens along the fold line because the fiber-to-fiber bonding along the fold gets loosened. Hence, once the tear is started, pulling the paper apart a little propagates the tip of the tear a little farther. Because of the loosened bonds, the pressure of tearing acts more effectively along this direction rather than getting diverted by the larger and stronger bonded fibres. So the tear follows the crease.

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.