Logic Gates: NOR

The NOR (= "not or") logic gate takes two bits as an input. Its output follows the rule: "if both bits are $0$, output $1$; otherwise, output $0$." You can implement it in your code as follows: given the Boolean variables $x$ and $y$, the symbol: $!(x || y)$ or $(x + y)'$ means "$x$ nor $y$". Below is the NOR gate diagram and truth table.

Bonus question: How can we build a NOR gate if we know how to build NOT, AND, OR, XOR, and NAND gates?