Masters have 13 mice to go through. Start by putting three mice in each side of the scale, with ten left over. -- If one side weighs less, the light mouse is in that group of three. -- If both sides weigh the same, the light mouse is in the group of ten, the ones you didn't weigh.

If you didn't find the group with the light mouse, repeat the process with the next six mice: put three in each pan (with one left over) and weigh them. -- If one side weighs less, the light mouse is in that group of three. -- If both sides weigh the same, the light mouse is the one you didn't weight (number 13).

If you found a group of three that's lighter, put one in each pan and put the third one down with the others (but apart, so you can remember which one it is). -- If one side weighs less, that's the light mouse. -- If both sides weigh the same, the light mouse is the one you put down with the others.