answers to stupid questions
I commend Cecil Adams's Straight Dope to you. For thirty-odd years they have been fielding difficult questions from the public with snarky answers. Reading the archives will probably teach you as much as a year in some schools.

There are, however, some questions they refuse to answer. In a spirit of homage, I'll take my lunchbreak to answer them here.

Why do we need a hot water heater? If it's hot it doesn't need to be heated.

Your grasp on the concept of apposition is shaky. Apposition is the placing of two nouns or noun phrases together, so that the one tells you more about the other. In some cases they are simply two ways to describe the same entity, like "my sister Lucy". You seem to be thinking that a "hot water heater" must be a heater to heat hot water, and that's one option, but consider other examples like "sausage machine", a machine to make sausages. A hot water heater is a heater which produces hot water.

How can we have jumbo shrimp?

This is a rather amusing question, because although the person asking it is presumably aware that "shrimp" is both the name of a creature and a description of size, they are possibly not aware that the same is true for "jumbo". Jumbo was a famous elephant in London Zoo in the 1880s; if he had been a particularly small elephant we could have said he was a shrimp Jumbo. Use of his name to mean "very large" followed within ten years. Shrimp are in fact so called because of their smallness (the word is related to "shrink") and we have records from the 1300s of using the term to describe people. So the fact that the adjectives in both cases are derived from comparing whatever it is to very large or very small animals is irrelevant; everyone knows you can have small elephants and big shrimp.

Why isn't phonetic spelled the way it sounds?

1) It is. The word is not irregularly spelt at all. (There are other ways of spelling the sound which "ph" makes, but that is irrelevant; the point is that "ph" can make no other regular sounds.)

2) Even if it wasn't, it is not necessary that a word should describe itself. This would lead us into shaving barber territory.

Why do our noses run and our feet smell?

This is a confusion about the two meanings of the verbs "run" and "smell". "Run" is a word which, in various forms, has meant "flow" since Proto-Indo-European; its use in describing the flow of your feet in bipedal motion and the flow of mucus from your nostrils are two applications of the same idea. "Smell" is one of those interesting words which can mean both sides of an action (compare dialectal "itch" for scratch and "learn" for teach); it's meant either to produce or notice an odour for almost a thousand years (probably longer, but we don't have attestation). There is an untrue story that Samuel Johnson, on being told, "Sir, you smell!", replied, "No, Sir, I stink, you smell."

Why does quicksand work slowly?

"Quick" here has the original meaning of "alive", as in "to judge the quick and the dead", and not the newer sense of "fast" (which has only been around since the fourteenth century).

Why are boxing rings square?

A lot of people seem to want to know this, for some reason. I think if you imagine a real fight between exactly two people, without all the formalised rules and layouts of boxing, the other people in the bar or marketplace or wherever tend to fall back into an approximate circle; people were using squares to fight in at least by 1743, when Jack Broughton's seminal rulebook on the conduct of bareknuckle fighting was published. I suspect it's simply easier to draw up a square on the floor, and it's easier to say that you start from opposite sides after a fall than saying you have to start from 180° from your opponent, but that's just a guess.

Why, when lights are out, they are invisible, but when the stars are out, they are visible?

"Out" is a word with a number of meanings, few of which can be descriptions of the same objects. You might as well ask whether Jodie Foster coming out meant she became more or less visible based on stellar or light analogies. If I still had access to the OED, I would tell you when the word came to mean "extinguished" when applied to flames (and hence electric light), but I don't.

[Update: Jonathan Jarrett, who does have OED access, shows me that the earliest record of the word being so used was by John Trevisa in his 1398 translation of De Proprietatibus Rerum: "For þat þe wynde schulde nouȝt blowe out þe light". You rock. Thanks.]

Why do we call them apartments when they are all together?

This question makes no sense because it is backwards. If you take a house and put all the parts together, it is precisely then when it is not apartments. If you split it apart into separate pieces, you have apartments.

If cows laughed, would milk come out of their noses?

I do rather find it hard to understand what kind of mental confusion would produce this question even as a joke. Firstly, when humans produce milk (or coffee, or whatever) from their nose when they laugh, it is because they were drinking it. Cows drink milk only in calfhood; the rest of their lives they drink water (around six gallons a day, although it varies by temperature). So only a baby cow would even be drinking the milk to start with. Secondly, why a cow? All mammals from mice to giraffes produce milk. Furthermore there is no connection between the mammary and the respiratory system. Finally, as far as I am aware, cows cannot laugh in the first place. Some zoologists have recently reported laughter in non-human primates, but not in other mammals.

Why does Denny's have locks on the door if it's open 24 hours?

A moment's thought should show you that the problems which would result from not being able to close the restaurant in an emergency, or in leaving it empty but unlocked, would far outweigh the few tens of dollars it costs to add locks to the door. Imagine, for example, closing the restaurant while refitting after a fire, or not being able to scrape enough staff together to run the place at 4am during a flu epidemic. (In addition, some jurisdictions may require that restaurants close on certain days or nights of the year.)

[Updates: rethought points out that a building may not have been open twenty-four hours a day in the past; shaunm points out that even buildings which are generally permanently open often close for a few hours a few times a year to have a cleaning crew do a thorough pass rather than the incremental cleaning that happens day-to-day.]

Why do ships carry cargoes and cars carry shipments?

"Car" (a Norman word which they took from the Celts) and "cargo" (a Spanish word that comes from Latin) are unrelated. Incidentally, it is interesting that people assume that the Welsh word for car, "car", is a loan-word from English, but its irregular plural belies this.

When will a building actually become a built?

The -ing on the end of "building" is not the "-ing" on the end of "singing"; it is another ending which was once "-ung" but merged. (The word in Scots is "biggin".)