How can I take a screenshot of the notification bar dropdown?

Sometimes I get a notification in the notification bar, I pull it down, and then I want to take a screenshot. I press the power button to make the screenshot, a menu appears to either shutdown, restart or make a screenshot. When this popup menu appears, the pulldown menu disappears. I've tried another screenshot app, but that didn't work either. **How can I take a screenshot of the notification pulldown menu?**