Why won't my iPhone i5 phone work when I turn it on its side?

Why won't my iPhone i5 phone work when I turn it on its side?

Its not called the "iPhone i5" for the record. Just the iPhone 5.

Have the screen rotation lock enabled? Or you could be in an app that doesn't support that.

Exactly as the first answer said, the app may not support it or you may have portrait orientation lock ON. To turn it off, if you have ios 7, slide up the control center and you could turn it off there. It's the toggle in the top right corner.