Program crashes by read two strings from keyboard

Question

I have a program that read 2 strings from keyboard and string1 will be replaced by string2. The problem is program crashes right after I press Enter. Can everybody explain what's wrong in my program? Thank you!

#include<stdio.h>
#define SIZE 80

void mystery1( char *s1, const char *s2 );

int main( void)
{
    char string1[ SIZE];
    char string2[ SIZE ];
    puts("Enter two strings: ");
    scanf_s("%79s%79s",string1,string2); // this line makes program crashed
    mystery1(string1, string2);
    printf_s("%s", string1);
}

// What does this function do?
void mystery1(char *s1, const char *s2 )
{
    while ( *s1 != '\0') {
        ++s1;
    }

    for( ; *s1 = *s2; ++s1, ++s2 ) {
        ;
    }
}

Answer 1

scanf("%79s%79s", string1, string2);

This solved the problem for me. Or may use:

scanf_s("%79s %79s", string1,80, string2, 80);

Answer 2

For "What does this function do?", the answer is, it combines both strings into string1 without carrying about overflow.

void mystery1(char *str1, char *str2) {
    while(*str1 != '\0') { // Is end of string check
        ++str1; // Move pointer position to next character.
    }

        // str1 now points to the first null character of the array.

    for(; *str1 = *str2; ++str1, ++str2) {
        // Sets the current str1 position to equal str2
        // str1 is incremented along with str2, but str1 is starting at the first null character
        ;
    }
}