Clear the iMessage app from your multi task bar. Do the four finger swipe up or press the home button twice which will reveal your bar across the bottom with all the apps you have opened. Press one of the apps until it jiggles and a red minus sign will appear above all of them. Press this minus sign and this will close the app, not delete it. It's a good idea to close all your apps regularly to keep the iPad and iphone running smoothly. Have a go now.
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