The economy grew at an impressive rate of four percent in the second quarter of this year, according to a government report released on Wednesday. But the stock market promptly tanked. The Dow lost more than 317 points Thursday and another 70 points Friday. What gives? Financial markets like it when the economy grows fast enough to signal that the recovery is continuing -- but not so fast that labor markets might tighten and workers get more bargaining power to get raises. Markets also worry that if the economy grows too fast, the Federal Reserve might pull back from its policy of low interest rates.