Why nand and nor are called universal gates?

NAND and NOR are called universal gates, you know. This is because we can make any other logic gate using them. Like, you can create NOT, AND, and OR gates. You can do this using only NAND gates. Or you can use only NOR gates. So, if you have only NAND gates, you can build any digital circuit you want. That is why they are so special. They are like the main building blocks for everything in digital electronics.