What is "You'll have someone's eye out with that"?

[Amazigh]

I came across this expression in this article: http://apps.facebook.com/thegua

[Mircat]

It's just a joke. I don't know if parents still do it but back in the day a parental rule was never run with something sharp in your hand like scissors, a knife, a pen, a pencil, a key, even a straw. It was always, "Stop that! You're going to fall and put out your eye!" So that was kind of an old fashioned saying that now people use as a joke, like someone carring a pillow someone might say be careful you'll put someone's eye out with that. It's usually said about something that wouldn't cause damage to make it funnier.