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.

For three straight days this week, the Dow Jones Industrials (DJINDICES:^DJI) have risen above 16,000 in early trading. But Dow couldn't hold those gains on Monday and Tuesday, and the average's oscillations continued this morning as investors await the afternoon release of minutes from the most recent Federal Open Market Committee meeting. As of 11 a.m. EST, the Dow was back below the key level, with its 11-point gain leaving it at 15,978. With Boeing (NYSE:BA) and Caterpillar (NYSE:CAT) both weighing on the index, investors have to wonder whether the Dow can finally hold this time or will stay below 16,000 a while longer.

Boeing led the Dow's decliners, falling 2.5% after analysts at Oppenheimer downgraded the aerospace giant. The analysts argued that recent news about huge aircraft orders have boosted the company's long-term prospects, but that continuing challenges in actually executing on designing and producing those planes might present problems that could send shares lower. Moreover, one element that some investors have ignored is that orders for newer aircraft models often replace demand for existing models, leaving Boeing with the question of how to manage current production lines and make smooth transitions to producing new planes.

Caterpillar dropped 1% after reporting that retail-dealer machine sales fell 12% worldwide in October. The Asia-Pacific region had the worst results, with sales plunging 26%, while North America held up the best with just a 2% decline. Sales of power systems dropped 9%, with particular weakness in the transportation and electric-power areas offsetting gains in Caterpillar's industrial segment. The news shows that Caterpillar's long-awaited recovery hasn't arrived yet and doesn't look likely to appear in the near future.

In the grand scheme of things, Dow 16,000 is an irrelevant milestone, yet until it's passed -- one way or another -- investors will still pay close attention to that number. Whether the Dow can climb through 16,000 for good could go a long way toward setting investor sentiment and determining which way the market moves in the months to come.

Fool contributor Dan Caplinger has no position in any stocks mentioned. You can follow him on Twitter @DanCaplinger. The Motley Fool has no position in any of the stocks mentioned. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.