Hi there I remember u posted a nice puzzle long back this one spl. for u : --> There are two persons. A pair of no is written on each's face. They can see the no (pair of no) written on the other's face and they have to tell the no written on their face. They start one by one. Lets say one person starts he can see the no written on his opponents face , its (13,17). After 3 tries he tells the correct answer. The nos exhibit the following properties: 1> All the nos are prime. 2> Any three no taken ( from the set of 4 nos, two no on each's face) together form a triangle and the perimeter of the triangle is prime. Now you have tell the no written on that persons face and why it took him three tries. The conversation between the two persons is as follows Let p1 and p2 be the persons and p1 starts and p1 saw the no (13,17) written in p2. p1> I don't know p2> I don't know p1> I don't know p2> I don't know p1> Tells the right no. enjoy (Happy X-mas) Rajiv PS : I'm off till 28th So u have enough time.