If you think the game Fruit Ninja is fun, check out this video. Someone actually played the game on Nokia Lumia 920 with a real knife. For most (if not all) smartphones, you will not be able to activate the capacitive screen with a knife. However, with Nokia Lumia 920′s sensitive touch screen, you can actually do that (but maybe not as fun as this).
Check out the video. Thanks to Gorilla Glass on Nokia Lumia 920, the screen should be able to survive it.