If you have ever sat on a tack by mistake, you'll know that tacks are very sharp. Used correctly, they are quite useful for posting notices on bulletin boards, for example.

However, if touched incorrectly, their sharpness can hurt you.

When we say that a person is "as sharp as a tack," we are admiring their wit and perception. That person, we are saying, is not only intelligent, but is able to make clever comments about most things in an insightful way. If such a clever person becomes arrogant or sarcastic, however, their words can hurt you.