what is a "Grape Nut"?

I've never known-- and my favorite reference to it was the hilarious SNL skit with Phil Hartman about "Colon Blow".

Here's Barry Newman in the WSJ with everything you ever wanted to know about Grape Nuts but were probably afraid to ask....

All the world's Grape Nuts come from a dirty-white, six-story concrete building with steam rising out of the roof here in the San Joaquin Valley. The valley grows lots of grapes and lots of nuts, so the factory's location would make sense, if Grape Nuts contained any local ingredients. Which it doesn't.

For 111 years, over breakfast, Americans have wondered: What's a grape nut? A grape nut looks like a kidney stone, but the name, unlike shredded wheat's, isn't self-descriptive. This raises many reasonable questions: Is it grapes that haven't developed? What part of the grape do they use? For those who have read the box and learned what Grape Nuts are made of (flour), a denser issue arises: How does a cereal with the mouthfeel of gravel get manufactured?...

When soaked in milk, the final product is mildly sweet brown stuff. But if Grape Nuts lovers don't know what the stuff is, Grape Nuts sellers have no doubts. Carin Gendell, who was its senior brand manager in the 1980s, remembers how her staff described it. "Grape Nuts," she says, "was people eating advertising."

Since people haven't been eating nearly as much of it as they used to, the latest Grape Nuts ad campaign, running now on MSN's Web site, is trying a new tonic: It consists of skits in which male milquetoasts get droll advice on "looking cool while driving a minivan," or "letting your in-laws move into your house." The slogan -- "That takes Grape Nuts" -- implies that the stuff enhances virility....

The founder of Postum Cereals not only cooked up Grape Nuts in Battle Creek, Mich., around 1898, but also concocted some of the earliest mass advertising to peddle it. A 1910 ad said Grape Nuts had "phosphate of potash" for building "brain and nerves." It didn't. Another said the Panama Canal couldn't have been dug without Grape Nuts because it "keeps almost indefinitely in any climate." Other ads claimed it prevented malaria and appendicitis. It doesn't.

By 1914, when Mr. Post apparently killed himself -- shortly after an appendicitis attack -- Grape Nuts had cut its curative claims to one: constipation....In the '70s...[if] Grape Nuts were wood chips, nobody minded -- the stuff was the seventh-biggest cold cereal in the land. But Mr. Gibbons died and the land became less earthy. Sales slid and never regained ground....

In 2005, four Grape Nuts ovens in Battle Creek were scrapped, leaving just the one here in California. With a share of the cereal market below 1%, the stuff was tilting toward crunchtime....

Mixed with yeast (one cup per 2,000 pounds) and water, the flour turns to dough, gets chopped into 10-pound loaves and sent into a huge oven -- 1,610 loaves at a time. "Now it gets interesting," Mr. Vargas said at his workstation, watching the loaves emerge from the oven and catapult into the darkness. An instant later, they hit the fan -- a whirling high-speed shredder that rips them to smithereens....The shreds dry for three hours. Then they're dumped onto rollers, crushed into crumbs, and poured through ducts down to the packaging floor -- 165,000 pounds a day, every day....

So what's a grape nut? Bread...