[battschr]

[ QUOTE ]
this puzzle is only solvable in 3 steps when using a balance scale.

and it is not too tough, but a bit fun.

separate into 3 piles of 4 rocks: A, B, and C

assume odd rock is light.

Place A and B on the balance. if balance = even, odd rock is in pile C. if uneven, odd rock is obviously on the lighter scale.

you now have a group of 4 rocks and one is odd, and can use the balance scale two more times.

Seperate this group of 4 into 2 groups of 2 stones: D and E.

again, lighter side contains the odd rock.

finally take these last 2 rocks, balance them on the scale. lighter rock is obviously the odd rock of the 12.

[/ QUOTE ]
Why can we assume that the odd rock is light?