A nxn grid is a latin square if each column and row contains the numbers 1 through n exactly once. If we divide a 9x9 latin square in nine 3x3 bocks and we want that each block contains the numbers 1 through 9 exactly once then we have a Su Doku. There are 6.67*10^21 Su Doku (A107739) out of 5.52*10^27 9x9 latin squares (A002860). Here is a Su Doku generator: each time you reload the page you get a new one.