Paradoxes

Just because I read yesterday an article about one paradoxe, I would like to list some today. Please notice that this list doesn't need to be complete.

Reductio ad absurdum:

A : All beliefs are of equal validity and cannot be denied.
B : If that's the case, then C is correct in his belief, even though C 
believes something that is considered to be wrong by most people, such
as that the Earth is flat.
A : True.
B : Then some beliefs can be denied. 

Liar paradox (which is also part of the discordianism):

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

Epimenides paradox:

Epimenides was a Cretan who made one immortal statement: 
"All Cretans are liars."

Quine's paradox:

"yields falsehood when preceded by its quotation" 
yields falsehood when preceded by its quotation.

Grelling-Nelson paradox:

Is 'heterological' a heterological word? 
If the answer is 'yes', 'heterological' is autological
(leading to a contradiction).
If the answer is 'no', 'heterological' is heterological
(again leading to a contradiction).

Curry's paradox:

If everything in this box is true, then Santa Claus exists.

However a very big list of paradoxes exists in the Wikipedia of course. Have a look at it and enjoy them.