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Press the Volume Up and Volume Down keys at the same time. Find your saved screenshot. Install a screenshot app on older Blackberry devices.
This will take screenshots on most Blackberry 10 phones, including the Q10, Z10, and Z30. Open the File Manager app and navigate to the Camera folder. You'll find your screenshots here. Most older Blackberry phones don't natively support screenshots. There are a few apps available on the Blackberry World market that will allow you to take screenshots on these devices, including:  Capture It Screen Muncher