How to forcibly close apps

On iOS

1. Click twice on the Home button and all multitasking apps will appear.
2. Swipe up on each app to remove it from running in the background.
3. Click the Home button to go back to the normal screen.
4. Now you can open the game again.

On later Apple models (without a Home button)

1. Swipe up from the bottom of the screen to return to your Home screen.
2. Swipe up again and hold your finger near the middle of the screen until all of your open apps appear.
3. Hold your finger in the middle of the screen a little longer to bring up a minus sign in the upper-left corner of the open apps.
4. Press the minus sign to close an app. You can also swipe up to close an app.

On Android

Please note: Instructions may vary slightly depending on your device or software version. 

1. Go to Settings → Applications → Manage Applications.
2. Select the app and tap Force stop.
3. Return to the Home screen and re-start the game app.