This blog post originally appeared on RealMoney.com on July 29 at 5:55 a.m. EDT.

NEW YORK ( TheStreet) -- It turns out copper was the metric.

Drats, I thought it was the dollar or oil. I thought we were supposed to buy the cyclicals on earnings being better than expected.

I thought we might be buying the minerals and the steels and the oils off the morning proxy of the Baltic Freight Index, known as the Baltic Fright Index in the days when it kept going down, and kept us out of the Freeports ( FCX) and Vales ( VALE) and Union Pacifics ( UNP) and U.S. Steels ( X).

Silly me.

We were simply waiting for two straight weeks of copper inventory builds over in London. That was the signal that China was kaput and you had to sell Chesapeake ( CHK) and BHP Billiton ( BHP)! Didn't get the memo.

And yet that's the universal one-minute takeaway of the Shanghai 7% plunge and will no doubt turn into another day of fruitful buying of Merck ( MRK), Johnson & Johnson ( JNJ), anything HMO and Express Scripts ( ESRX), not to mention longtime Cramer faves Hershey ( HSY) and Altria ( MO).

How about tech?

China.

Could get hurt, too.

(There's also a story that Chinese banks are going to cut down lending, but that's a story that I have seen before, making the copper-LME story even more cogent.)

The binary nature of the market repulses us because 1) it has nothing to do with us, as we don't set the price of oil, China does -- unless of course we manipulate it -- and 2) we are a tenth of the world's copper use and they are a third of it.

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