package challengestrings;
import java.util.Scanner;
public class maths {
public static void main(String[] args) {
// TODO Auto-generated method stub
Scanner scn = new Scanner(System.in);
int t = scn.nextInt();
for(int i=1 ; i<=t ; i++)
{
String N1=scn.next();
String N2 = scn.next();
System.out.print(maths(N1,N2));
}
}
public static String maths(String N1 , String N2)
{
String N3="";
for(int i=0 ; i<N1.length() ; i++)
{
if(N1.charAt(i)==N2.charAt(i))
{
N3=N3+0;
}
else{
N3=N3+1;
}
}
return N3;
}
}