How and where do you find where a stock is trading after hours?

[MrBojangles]

Sometimes I see that a stock is dropping or rising in price after the market is closed? It shows this on yahoo finance. How is this possible? Is there a cut off point? Could I watch what twitter was trading at over a weekend?

[Mopes]

Some stocks can be bought or sold 2 hours before/ after market hours. You have to go to a/ your broker's site and then sign into after-hours trading to get quotes. It is considered riskier trading and has some limitations versus normal hour trading.