1. If unicorns existed, the reason they would not exist is because of extinction (Dinosaurs once existed...no longer the case today).
2. Laughter at the bare minimum is pronounciation from the vocal cords like any other noise you can produce. It is associated with the expression of happiness or reaction to being tickled as that is a common human response to such items.
3. In a world of red, blue is not defined and does not exist. In other terms, it means nothing...it's just an arrangement of letters. It is possible for blue to be assigned another meaning such as feeling the "blues" in which case the core word "blue" means depressed.
4. Aside from the obvious Chuck Norris meme, I have not an answer for this one. I'll take LM's answer of a river
5. One. Fire is a chemical reaction and thus will not take away from the original chemical reaction of the torch as it spreads to the million candles.
6. The letters that have 2 perpendicular lines of symmetry required for all conditions:
H, I, O, S, X, Z
The only ones I can construct is "SOS", and "I"
7. You do not have enough force and matter to produce a sound that is audible. Furthermore, there is no density for sound to travel through solid objects in your body such as bones.
8. The concept of stereo vision. It's brain can process two independent images and determin the location and distance of an object within it's eyesight range.
9. A word that contains an infinite amount of letters (Division by Zero error must be created).