In today's world we all seem to use mobile data every single day. Whether we are just browsing the net, checking twitter, uploading pictures to facebook, streaming live video of Japanese TV (yes, yes, I know that's just me!) or using an instant messenger like Skype or Whatsapp.

Now a days however we get plenty of chances to log on to a Wi-fi network, be it our own in our own houses, one we have access to through our service provider or a general one in a restaurant, cafe or hotel. When you can get a Wi-Fi signal it's usually a better (read "faster" and "more stable") connection than you'd get using mobile data so it's advised to turn off your mobile data to save battery power whilst connected to Wi-fi.
With the Samsung Galaxy S2 (and various other Androids) there is actually 2 ways in which a user can turn off their mobile internet:
Method 1-If you drag down the notification bar and scroll across slightly you will get the option of "Mobile data" if you press on this and turn the light off this will turn off mobile data and save the battery. (For reference see above).

Method 2-Alternatively if you hold the power button you will get a "Device options" screen (see the picture to the right) which allows you to turn off "Date network mode" which is actually the same thing.

To turn it back on you can just do every thing again but make sure the light is "on" in the notification panel or the option says "activated".

For most of he time the notification bar method is the easiest and quickest, but for the few occasions where it may be difficult to access the bar, the  "Device options" method may be an easier alternative.