"Unfortunately CrystalBall has stopped"

Can you post the content of LogCat - the answer will probably lie in there somewhere!

I had a few instances of this happening - have you done Project | Clean ... that might help.

Steve.

Thanks. I tried Project | Clean but it didn't help. I'm new to this... Here's the log, It's a little long...

06-11 12:01:52.145: D/AndroidRuntime(1411): Shutting down VM 06-11 12:01:52.145: W/dalvikvm(1411): threadid=1: thread exiting with uncaught exception (group=0xa4d00b20) 06-11 12:01:52.161: E/AndroidRuntime(1411): FATAL EXCEPTION: main 06-11 12:01:52.161: E/AndroidRuntime(1411): Process: com.example.crystalball, PID: 1411 06-11 12:01:52.161: E/AndroidRuntime(1411): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.crystalball/com.example.crystalball.MainActivity}: java.lang.IllegalStateException: You need to use a Theme.AppCompat theme (or descendant) with this activity. 06-11 12:01:52.161: E/AndroidRuntime(1411): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195) 06-11 12:01:52.161: E/AndroidRuntime(1411): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245) 06-11 12:01:52.161: E/AndroidRuntime(1411): at android.app.ActivityThread.access$800(ActivityThread.java:135) 06-11 12:01:52.161: E/AndroidRuntime(1411): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196) 06-11 12:01:52.161: E/AndroidRuntime(1411): at android.os.Handler.dispatchMessage(Handler.java:102) 06-11 12:01:52.161: E/AndroidRuntime(1411): at android.os.Looper.loop(Looper.java:136) 06-11 12:01:52.161: E/AndroidRuntime(1411): at android.app.ActivityThread.main(ActivityThread.java:5017) 06-11 12:01:52.161: E/AndroidRuntime(1411): at java.lang.reflect.Method.invokeNative(Native Method) 06-11 12:01:52.161: E/AndroidRuntime(1411): at java.lang.reflect.Method.invoke(Method.java:515) 06-11 12:01:52.161: E/AndroidRuntime(1411): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779) 06-11 12:01:52.161: E/AndroidRuntime(1411): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595) 06-11 12:01:52.161: E/AndroidRuntime(1411): at dalvik.system.NativeStart.main(Native Method) 06-11 12:01:52.161: E/AndroidRuntime(1411): Caused by: java.lang.IllegalStateException: You need to use a Theme.AppCompat theme (or descendant) with this activity. 06-11 12:01:52.161: E/AndroidRuntime(1411): at android.support.v7.app.ActionBarActivityDelegate.onCreate(ActionBarActivityDelegate.java:111) 06-11 12:01:52.161: E/AndroidRuntime(1411): at android.support.v7.app.ActionBarActivityDelegateICS.onCreate(ActionBarActivityDelegateICS.java:58) 06-11 12:01:52.161: E/AndroidRuntime(1411): at android.support.v7.app.ActionBarActivity.onCreate(ActionBarActivity.java:98) 06-11 12:01:52.161: E/AndroidRuntime(1411): at com.example.crystalball.MainActivity.onCreate(MainActivity.java:19) 06-11 12:01:52.161: E/AndroidRuntime(1411): at android.app.Activity.performCreate(Activity.java:5231) 06-11 12:01:52.161: E/AndroidRuntime(1411): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087) 06-11 12:01:52.161: E/AndroidRuntime(1411): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159) 06-11 12:01:52.161: E/AndroidRuntime(1411): ... 11 more 06-11 12:01:59.517: I/Process(1411): Sending signal. PID: 1411 SIG: 9

That looks like the Theme is set incorrectly. I'm not sure about how to fix that (assuming I'm right!). Do you have appcompat_v7` at the top of your Package Explorer folder list?

Steve.

What's on line 19 of your MainActivity?

Oh! man thanks for your time! I've noticed there was a known issue about that, because of an update of the Android Tools. I've followed the "Known Issues" below the video, and it worked. I guess the main problem was changing the theme to Theme.Black.NoTitleBar.Fullscreen instead of appTheme