Rationality:

Much  of game  theory  assumes  that the players  are  perfect  calculators and flawless follower  of the best  strategies. This  is  the assumption of rational behaviour. Rationality means that each player has a consistent ranking over all the logically possible outcomes, and calculates the strategy that best serves these interests. Thus, rationality has two essential ingredients:  complete knowledge of one's  own.interest and  flawless calculation of what actions will best serve those interests. 

Common Knowledge:

For some facts,or  situation, say, X, to be common knowledge to the players A and B, it  is not enough to know X separately. Each should also know that the other knows X. Otherwise, for example, A might think that B does not know X, and  the other way around. A  should know that B knows that A knows that B  knows,  and  so on  ad  infinitum. This concept  in  game theory  is  known as common knowledge.