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: Exelixis (Nasdaq: EXEL) shares jumped 12% today on high volume.

So what: Momentum traders have gotten a hold of this stock, pushing it consistently higher throughout the day today. Just before close, volume stood at 3.4 million shares, far exceeding the average volume of 1.4 million shares.

Now what: There are also rumors of a buyout swirling, but nothing concrete, so it could be a move on leaked information. Regardless, this momentum traders' dream poses a lot of risks for the long-term investor who isn't watching the stock every minute. I'm going to stay away for today and wait for some concrete news before making a move.

Interested in more info on Exelixis? Add it to your watchlist.

Fool contributor Travis Hoium does not have a position in any company mentioned. You can follow Travis on Twitter at @FlushDrawFool, check out his personal stock holdings or follow his CAPS picks at TMFFlushDraw.

Exelixis is a Motley Fool Rule Breakers selection. The Fool owns shares of Exelixis. 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.