Why are NAND and NOR gates called universal gates?

NAND and NOR are universal gates because any Boolean function can be implemented using only NAND gates or only NOR gates. Using NAND: NOT is NAND with tied inputs, AND is NAND followed by NOT (another NAND), OR is NAND of inverted inputs. Similarly for NOR gates. This property makes them fundamental in IC design as manufacturing can be simplified by using only one gate type.