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

What: Shares of NYSE Euronext (NYSE: NYX) -- you know, the folks who run the stock market? -- dropped 11% this morning.

So what: Why? Across the pond, Chancellor Angela Merkel of Germany and French President Nicolas Sarkozy just emerged from a discussion of who's going to pay for all of Europe's debt. The answer? You guessed it: "NYSE Euronext." And InterContinental Exchange (NYSE: ICE). And Nasdaq OMX (Nasdaq: NDAQ). And and CME Group (NYSE: CME). The twin heads of state are proposing a new tax on financial transactions in Europe as a way to start chipping away at the continent's debts, scaring the bejeezus out of anyone who thinks his or her business might be "financial," or "in Europe."

Now what: Today's Wall Street Journal tells us there's no need to get upset about this and that Europe has mulled similar taxes in the past -- to zero effect. But I wonder whether this time is different. I mean, in the United States, Congress is now talking about possibly taxing home mortgage interest. That's considerably more sacred to Americans than capitalist profits are to European socialists.

Seems to me everything's on the table today -- and financial investors are right to be scared.

Will Europe carry through on its threat? Add NYSE Euronext to your Fool Watchlist, and keep up to date on developments.

Fool contributor Rich Smith does not own (or short) shares of any company named above. Motley Fool newsletter services have recommended buying shares of NYSE Euronext. We Fools don't 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.