In philosophy and logic, the liar paradox encompasses paradoxical statements such as:

* I am lying now.
* This statement is false.

These statements are paradoxical because there is no way to assign them a consistent truth value. Consider that if This statement is false. is true, then what it says is the case; but what it says is that it is false, hence it is false. On the other hand, if it is false, then what it says is not the case; thus, since it says that it is false, it must be true.

To avoid having a sentence directly refer to its own truth value, one can also construct the paradox as follows:

The following sentence is true. The preceding sentence is false.

However, it is arguable that this reformulation is little more than a syntactic expansion. The idea is that neither sentence accomplishes the paradox without precisely its counterpart...

(quoted from wikipedia)
Link: http://en.wikipedia.org/wiki/Liar_paradox