How do you take a screen shot on the samsung galaxy s 4g?

I have been trying to figure out how to take a screen shot on the Samsung Galaxy s 4g without downloading an app. Everybody keeps saying you have to have android 2.2 to take a screen shot what is android 2.2 and how do you take a screen shot?How do you take a screen shot on the samsung galaxy s 4g?you can get an app to do that, there are many apps in the android app market.