Although we don't believe in timing the market or panicking over daily movements, we do like to keep an eye on market changes -- just in case they're material to our investing thesis.

The stock market resumed its slide today, as gains from yesterday's small rebound were erased before the ringing of the opening bell. Ever since data showed Chinese manufacturing unexpectedly contracting last week, Wall Street's been hypersensitive to overseas developments. Currencies in emerging markets like Turkey, India, and South Africa have been losing value since then, causing problems for globally diversified investors. Not only does it make U.S. exports less attractive, but domestic companies will notice their foreign profits start to dwindle as conversion rates make the dollar more and more expensive. Embracing their fears, investors sent the S&P 500 Index (SNPINDEX:^GSPC) down 18 points, or 1%, to end at 1,774. 

But if you think the benchmark S&P 500 Index has been having problems over the past week, you haven't been paying attention to Yahoo! (NASDAQ:YHOO) stock, which cratered 8.7% Wednesday. Shares have now lost 13.2% in the last five trading days alone, far worse than the S&P's 3.8% hit over the same period. Today's sell-off was triggered by Yahoo!'s disappointing quarterly revenue figures, which the company reported yesterday. Not only did display ad sales post a 6% decline, but the sales growth of China's e-commerce giant Alibaba is cooling down, too. This is bad news for Yahoo!, which owns 24% of Alibaba, and still relies on Alibaba as an engine for growth. 

That old Wall Street adage that "investors buy the future, not the past" has never been truer than it was today with shares of Boeing (NYSE:BA), which shed 5.3%. Its profits surged by nearly 30% in the most recent quarter, as the aerospace behemoth set new delivery records for three different airplane models. But most large investors like consistency and some degree of predictability in a company's progression, so when Boeing said its earnings would grow by only about 2% this year, the stock tanked. The stock had a great year in 2013 and it can't keep growing at a 30% annual pace forever, so one of these days Boeing's multiple will come back down to earth -- I just don't want to be on board when it does.

Lastly, $35 billion Houston-based energy transportation company Kinder Morgan (NYSE:KMI) saw shares slump 3.9% Wednesday. The stock, like all others in the market today, was facing an uphill battle against fearful money managers as investors shifted their assets to lower-risk investments. But on top of that, Kinder Morgan stock started trading "ex-dividend" Wednesday, meaning anyone who owned a share yesterday could have sold today and still gotten the next quarterly dividend payment. Ex-dividend days give a downside bias to the stock for that day, since many short-term investors elect to sell their shares immediately and lock in that dividend. Longer-term investors (like you!) should ignore these trivial fluctuations.