1 Obi-Wan: Your father's lightsabre. This is the weapon of a Jedi Knight.
2 Obi-Wan: {as Luke activates the lightsabre} Not as clumsy or as random as a blaster. An elegant weapon for a more civilised time.
3 [sound]: Whoom!
3 Luke: Yow! {cuts his own hand off}
4 Obi-Wan: Yes, well... As long as you don't put your hand too near the blade.

2013-01-07 Rerun commentary: Why not start all the hand-cutting-off a bit earlier?

In some ways it's curious that Luke doesn't actually touch the blade in this scene. From the way Obi-Wan describes the lightsabre, it's pretty clear that Luke has never seen or heard of one before, so he doesn't know that the blade will instantly destroy anything it touches. I guess the glowing and humming is enough to make him a bit wary.

But still, someone hands you a regular sword, what's the very first thing you do? You feel the blade! This scene could very easily have gone tragically wrong.