Why NAND ,NOR gate called as universal gate ?

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Q- WHY NAND , NOR GATE CALLED AS UNIVERSAL GATE ?

A- The reason is very simple , we know we have ( not , or , and , nand , nor , exor , exnor ) gates , NOT , AND , OR are the BASIC GATE  , by basic gate we can make any digital circuit , but this digital circuit make by combination of these three gates ,any of these three  indivuasly  can not make the digital circuit .  but word UNIVERSAL means " everythink " and in diigital electronics that everything is " logic gates " . so there is any gate which indivisually make any digital circuit ? answer is yes .  NAND , NOR gate indivisually make any digital circuit . they can make any BASIC GATE ( not , and , or ) . then as we know basic gate makes any digital circuit . that means we can directly say that NAND , NOR can make any digital circuit indivisualy without combination of any other gate . that's why we called these two gate as a universal gate .