Why my code is showing this kind of behavior?

```
#include <stdio.h>

void s1()
{
    int a;
    a++;
    printf("%d ",a);
}
void s2()
{
    int aa;
    aa++;
    printf("%d ",aa);
}

int main()
{
    int i = 0;
    for(i = 0; i < 10; i++)
    {
        s1();
    }
    for(i = 0; i < 10; i++)
    {
        s2();
    }

    return 0;

}

```

Output: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Why 'a' in second function is storing value from first function?