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We'll assume you mean that the app is closing on you and not actually turning off. Either way we need more information.
What device do you have?
Is there a fault or message that appears when the app closes?
Is this the only app you've noticed closing like this?
Have you tried restarting your phone? Simple restarts can help with minor issues and regular restarts can keep your device running smoothly.
How long does it take for the app to close? Does it open and immediately close, it opens and operates for a brief time then closes, it is random it might run for minutes and other times it opens and closes right away, what is happening?
You could try clearing the app cache and app data to see if that helps. Not knowing what device you have I can only give you general instructions, Settings>Applications>Calculator>Storage>Clear cache / Clear data