How do you know when to harvest corn? The stuff I've read talks about
moisture content. What exactly is that? How do I measure it without
buying a scientific device? Or is there another way? I've got a
backyard garden, with about 15 corn plants. I'm not going to buy
expensive farm equipment for a garden this size.

I think you're thinking of harvesting field corn, which is allowed to dry down
before combining. Did it say something about 15% moisture? If so, you're
talking fully matured corn, not sweetcorn, which is eaten quite young.

I've also heard people say that you can just open up an ear and look at
it. What do I look for? When?

Actually, sweetcorn tends to be at its best about 2 days after the rackety-
goons get it. g

A fairly reliable indicator for folks who know *nothing* about sweetcorn
is to harvest about 20 days after the silks appear. Other clues are that
the silks are starting to brown almost all the way to the cob, the ear
has started to firm up in its husks, and the kernels toward the "open" end
of the cob start feeling fairly good sized (the very tip rarely fills out
completely). However, if your corn isn't getting 6 hrs of sun a day,
or if the pollination was poor, those are not going to be quite as reliable
indications as if it were grown under "standard" conditions.

The real test is to open the husks a bit and punch a kernel with a thumbnail.
If you get a milky fluid out, it's pretty much ready to eat. If you get
something that looks pretty doughy, it's past prime (you get about 1 week
of the milk stage at summer temps for most corns.)

There are some pictures here that might be helpful:
http://www.agry.purdue.edu/ext/corn/...fill-0717.html
You want the "R3" stage.

Even more helpful would be to stop by a local farmstand, farmers market, or
grocery store with a knowledgeable produce manager, and ask them to help you
choose some corn, and show you what you're looking for. (I'm an Iowan
transplanted to Oregon, and I seem to amaze folks by sorting ears of
various maturities without having to open the husks -- it really just takes
some experience, and once you've learned, it's like riding a bicycle.)

Unless you've planted SE or Supersweet corns, it's good to harvest just before
you cook it -- within an hour or so of cooking. It's not necessary to start
the water boiling before you go out to pick and run back with the harvest,
shucking as you go, but that's sure good corn when you do. g

Kay