Coding Puzzle No. 2?

#include <stdio.h>
#define x 10

void foo();

void main()
{
        printf("%d\n",x);
        foo();
        printf("%d\n",x);
}

void foo()
{
        #undef x
        #define x 20
}




Output:
10
10

ConCept:
 During preprocessing, the compiler checks for preprocessor directive sequential, it's that time when code is not yet compiled, so both 'x' are replaced by 10.