Windows Phone 8.1 textblock doesnt scroll if outside screen.

Question

Hi,
I'm developing an app for windows phone 8.1
How do I make a textblock scroll?
When I have text that's longer than the screen it doesn't scroll down.
Example.

I understand the scrollviewer function but I dont know how to bind to the phone height

<Scrollviewer>

<Textblock Text="lorem ipsum...." TextWrapping="Wrap" />

</Scrollviewer>