NAND and NOR gates are universal gates. Prove it by realizing all the following gates:
NOT
AND
OR
XOR (Exclusive OR gate)
X-NOR (Equivalence gate)
NOR (using NAND gates)
NAND (using NOR gates)
You have to realize each of the gates twice (except for NOR and NAND), once by using NAND alone and next by using NOR gate alone.