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You don't need to close apps, that's why there is no 'X'. They are garbage-collected as needed.
Yes, but, how do you return to the main desktop (that Metro thing) when you are running a Metro app just using the mouse?.. not the keyboard or the windows key.
Hover over the bottom left and the start icon becomes visible with a list of Windows currently open.
Move mouse to bottom right or top right. Menu will appear on the right (charms).. there is button there in the middle that you can click to get back to start screen..