U.S. stock markets have been on a tear so far this year, making it more than understandable that investors might be thinking about some possible selling. One such stock that certainly fits the bill is materials specialist Corning (GLW -0.10%). Up around 30% so far in 2012, the company certainly isn't as cheap as it once was, but does that make it a sell? In the most recent edition of our "Ask a Fool" series, Fool contributor Andrew Tonner weighs on in whether he thinks Corning is worth holding onto today.