Peanuts!

All right folks, it's time to learn about where food comes from!
Specifically today, it's peanuts.
Peanuts come from the ground!  And the sun!  Like all food, when you get down to it, including meat, since animals eat plants.  But really.
As you no doubt vaguely remember from school, a peanut is a legume.  A simpler word would be "bean."  Yes, peanuts are beans.  So are chickpeas and favas and snow peas; hummus is just bean dip with a better agent.  Peanuts are unique in the bean world, though, because they're more or less the only bean that grows underground.  (Okay, so, there are about 70 species in the peanut genus, some of which share the peanut's frankly bizarre growth habits, and one or two from related genera, but that's it.  There are over 19,000 species of beans.)
Say you want some peanuts and there are no stores anywhere around because of a recent zombie apocalypse (which has apparently since subsided, but that's another story).  But luckily you happen to have a raw peanut (why?  I don't know; again, there's clearly more to this story).  (Also, this doesn't work with a roasted peanut.  Cooked seeds mostly don't grow very well...although I once had an ash tree seed germinate after it went through the dryer in the pocket of my shorts so maybe this needs to be investigated more thoroughly.)

So anyway, you stick that raw peanut in the ground.  You should shell it first; wild peanut ancestors have much thinner shells that allow water in and the germinating seedling out, but we've bred peanuts for tougher shells so we can roast them and ship them and eat them at ballgames and litter the floors of "roadhouse" style restaurants with them.
If the ground is warm enough, the peanut will germinate and grow.  It's a pretty little plant.  After a few weeks, it will start flowering.  These pretty yellow flowers are a nice treat.  Notice how many there in just a small area!
They don't have much smell.  Anyway, an individual flower is only open for a day.  After pollination (it's self-fertile, so if the zombie apocalypse in this scenario also affected bees, you're still okay), the flower drops off, and the stem becomes what's called a 'peg,' a stiff downward pointing stem with a slightly hardened tip.  This tip penetrates the soil--hopefully you planted it is some nice sandy loam, and not clay--and once the plant perceives that it is below ground, the end of the peg starts to swell.  (Yes, plants perceive whether they are receiving light or not, but not in a way you'd recognize as 'seeing' and not in a conscious sense.  It's all electrobiochemistry and fairies.)
The peg, you see, had a secret--it was actually the peanut ovary.  You just wouldn't have noticed because it was tiny.
Once below ground, the ovary swells into a little peanut, and eventually into a big peanut.  The plant will continue to flower and produce new pegs and peanuts for quite a while, as long as it stays sunny and warm.  As autumn comes on it will stop flowering as much, and gradually start to die back; peanuts are annuals and don't live through the winter.  But if you wait for the plant to die, you're too late: most of the peanut shells will have succumbed to the constant assault of water and microbes that life in the soil entails, and you'll have mostly a bunch of rotten garbage.  So you want to harvest in mid-Autumn sometime, before any frost but after the bulk of the heat of summer is past.

And this is what you'll get: a bunch of peanuts in various stages of ripeness.
Generally if the peanut looks like a peanut, it's edible.  In this picture there are some little proto-peanuts, but mostly good mature peanuts.  You can see how they are attached to the pegs, and the pegs are attached to the stems of the plant.  So there you have it: peanuts grow below ground, like potatoes and rutabagas.  (Not that they are at all related to either of those things.)
You may wonder, why on Earth does the peanut do this to itself?  I mean, I have some perfectly nice runner beans out there growing on vines, flowering and producing pods right out in the sunshine, and even some nice bush butterpeas doing the same thing.  They seem to be just fine.
The truth is nobody knows why.  It's just what peanuts do.  For whatever reason the progenitor of the Arachis genus found it beneficial to grow this way; perhaps some predatory animal ate all the peanuts that weren't underground, gradually selecting for this growth habit.
What we do know is that the current version of the peanut, Arachis hypogaea, arose from wild progenitors in what is now northwestern Argentina, probably 8000-9000 years ago.  The wild relative can still be found in the Chaco area, but it's a very different plant.  Your domesticated peanut will have had a nice compact form, although low to the ground and gradually spreading--if you only planted the one, and it was a happy plant (like mine), maybe it would cover about a 3 foot diameter area.  In cultivation, they're planted in rows 30" apart, with individual plants about a foot apart in the rows.  This keeps them nice and compact and allows them to completely cover the soil and choke out weeds.  The wild relative is a vine that rambles along the ground and drops a peg into the soil every few inches; the nuts are much smaller, and almost exclusively come one to a pod (which are called unipeas.  Yes, they are; when there's only one seed in the peanut pod it's a unipea and nobody can tell me otherwise).  The shells are also thinner with heavier webbing.  But that's what a few thousand years of domestication will do for you.

Once harvest time comes around you basically have to go in there by hand and carefully, gently pull the plant out of the ground.  Mine came out in two parts.  I left about a dozen peanuts behind, but was able to sift through the soil to find them.  For many many years in the South kids could get a couple weeks work in the fall harvesting peanuts by hand, but the engineers have figured out a way to mechanize the process without leaving too many peanuts behind.  Since you only have the one plant--and in the post-zombie landscape you probably aren't going to find a working peanut harvester, much less someone competent to operate it--you'll dig yours up by hand, turn it upside down, and leave it in the sun for a day or two.  Then you can pop the little peanuts off and toss the plant in the compost heap for next year.
You'll want to roast them, of course, unless you'd rather boil them.
You'd rather boil them.
If you aren't familiar with boiled peanuts, what are you doing with your life?  Get down South and try some!  In any event you'll want to process them somehow before you eat them, although they are edible raw.  Be sure to set a few aside before you boil them, so you can plant them again next year.
(Note: in reality, if you started your peanut farm with a single peanut, you'd create what's called a genetic bottleneck: your entire farm would consist of plants that had only those gene versions--called alleles--that were present in your first nut.  This is a recipe for disaster, since you've only got two possible resistance genes for any given disease (and more likely you have none), and the whole population will be highly susceptible to any disease or insect that comes along.  This is called a 'monoculture', a huge field of genetically identical plants, and it's not a very smart way to ensure your survival as a species.  Of course after the zombie apocalypse humanity itself will have gone through a pretty severe genetic bottleneck.)
Incidentally, peanuts are a super crop to grow once in a while if you have a garden or just some unused land (note: lawns count as unused land.  You do nothing but spend money on it.  At least put some plants out there that will give you something back for your time and expense).  Why?  Well, like all beans (and certain other plants), peanuts are best friends with some little bacteria that normally live quiet, boring lives in the soil.  But when a peanut plant sends down roots, the roots exude a chemical that signals to the bored little microbes that it's time to party.  They associate with the root, and the peanut plant sends some tasty carbohydrates their way.  They feast on these, and in return, they take nitrogen in the soil and atmosphere and turn it from a boring, useless gas (which really doesn't do anything for your tires that regular air won't do, regardless of what the ads say) into ammonia.  Plants can't do that by themselves; neither can animals.  In order to get nitrogen into your body--and without nitrogen, you can't make any proteins, enzymes, even your DNA--it first has to be fixed into ammonia by these little microbes.  Much of the nitrogen they fix gets absorbed by the peanut plant (which is why you compost the plant rather than putting at the the curb in a garbage bag), although plenty of it stays behind in the soil when the microbes die, and is left there for other microbes and plants and soil-dwelling animals, which is how it gets into the food supply.  This picture is of a peanut root--all those little bumps are called nodules.  Each nodule grows from a single root hair, which is colonized by several microbes.  Each nodule is a protective home for a batch of microbes as they churn out ammonia.
So there you have it.  Peanuts!
Next time: pumpkins!