Hi I have solved some grundy number problems in contest and for all of them I have always use this standard straight forward approach of finding the lowest number which is not in the set. I would like to know what does a grundy number tell us... Is it the number of possible moves which allow someone to from that position.. and why do we take the lowest number not in the set as the grundy number.. y xor all sub games?? Everywhere I see the pseudo code and implementation but no explanation to why this works.. can anyone please shed some light... thanks