I read an interesting post the other day asking the hypothetical question of what do to if a stock you own goes up 20% in a single day. The answer given, of course, is "it depends" and I couldn't agree more. Every situation is different and should be approached as such.

I really got a kick out of the comments. I was surprised to see the fire and emotion this simple hypothetical situation evoked.

Then again, should I have been so surprised? The author's topic combined two equally volatile ingredients, greed and the element of surprise. It proved a potent recipe for eliciting that most basic instinct of self-preservation: the fight or flight response.

But I don't intend to go about analyzing reactions to the post. You should take a few minutes and read it for yourself. Then ask yourself what you'd do. Your answer says a lot about the type of investor you are.

Want to know what I'd do?

Nine times out of ten I'm selling. The one time I wouldn't be a seller is when I think the market is finally realizing why I bought the stock in the first place. The other nine times I'm selling because there is something going on that neither I nor all those buyers knew about.

Nothing like that second and much more likely scenario takes place (9 out of 10? That's fairly optimistic, wouldn't you say?) without a reason. There has to be a reason, some sort of fundamental change in structure or environment. Something. I'm a believer that change is usually a good thing over the long haul if it's implemented for the right reasons. Near term, especially with companies made up of people like you and me, change - even good change, the kind that sends the stock up 20% in a day - is hard to deal with.

Almost without exception, I'd take my money off the table, survey and evaluate the situation. Come tomorrow I might look like a loser, but I'd be a goat with a twenty point gain with no risk of becoming the hero with only a ten point gain.

What would you do if your stock went up 20% in a single day?