The number of the question is already a hint: 13. The code used for encoding is the so called "ROT-13" code.
The first 13 letters of the alphabet are shifted plus 13 while the last 13 letters are shifted minus 13:
abcdefghijklm
|||||||||||||
nopqrstuvwxyz
Applied on the message the following occurs:
oY ura eevyrs amtr
Additionally you have to exchange the letters pairwise (spaces included):
You are very smart
