Gates: truth tables
  NOT   AND OR XOR NOR NAND XNOR
a b ~a ~b a & b a | b a ^ b ~(a | b) ~(a & b) ~(a ^ b)
0 0 1 1 0 0 0 1 1 1
0 1 1 0 0 1 1 0 1 0
1 0 0 1 0 1 1 0 1 0
1 1 0 0 1 1 0 0 0 1
 
How many possible boolean functions of 2 variables?
Depends on number of outputs
4 possible inputs --> 16 possible outputs of 1 bit each