In this article is explained How to handle mouse event on entire Grid Control in WPF or Windows Store apps or Windows Phone apps.
It is really simple and I would not write anything about that in my blog if I had not had problems with this simple action in my Windows Phone project that I am working.
At the bottom of this article it says the gold rule:
In order for this to work you need to always set the Grid Background, either to transparent or to whatever you like, because if its null the event isn’t triggered…
And that’s true, remember to put a background on your Grid, by default could be set to null.