Time to go to Hogwarts

[vonflare]

I agree with the same-boat idea, and once on, I complain that 4 to a boat means we cannot play mafia on the boats.

Once Inferno, BuJaber, and the fourth person (if any) board, I ask the following puzzle:

You have 13 identical magic stones, except that at most one of them has an evil curse of bad luck. You remember being told that the cursed stone will weigh slightly more... or was it less... Anyway, you have a balance scale that will work exactly 3 times, and you also have another stone with the same weight as a non-cursed stone.
Your task is to find the cursed stone before... bad things happen? And because of the bad luck curse, the evil stone will probably be the one that fools your strategy, if possible.

I number the stones 1 to 13

I weigh stones 1-4 against stones 5-8

If the scale is balanced we know the evil stone is within stones 9-13.

Weigh 9+10 against 11+1. (we know stone 1 is not evil at this point because it was on the original scale)

If the scale is balanced, we know 12 or 13 is evil, and we can weigh 12 against 1 to determine whether it is 12 (if the scale is imbalanced) or 13 (if it balances).

if 9+10 vs 11+1 is imbalanced, I weigh 9 against 10. If it balances, 11 is the evil stone.

If it isn't balanced, 9 or 10 is the evil stone, and it can be deduced which one by whether 9 or 10 is heavier/lighter:

If in the 9+10 vs 11+1 weighing, if 9+10 was heavier than the evil stone is whichever one is heavier, and if 9+10 was lighter than the evil stone is whichever one is lighter.

____________________


If the original 1-4 vs 5-8 scale is unbalanced, then the evil stone is within 1-8.

Relabel the stones: the ones on the lighter side are L1 L2 L3 L4 and the ones on the heavier side are H1 H2 H3 H4

weigh L1 + L2 + H1 against L3 + H2 + stone 13 (which we know is not evil)

If the scale is balanced, either L4, H3, or H4 is the evil stone.

Weigh H3 against H4

If scale is balanced, L4 is evil. If scale is unbalanced, the heavier stone is evil since H3 and H4 were on the heavier side of the original scale.

if L1 + L2 + H1 is heavier than L3 + H2 + 13:

given the conditions above, L1, L2, H2, and 13 are not evil. That leaves only L3 or H1 to be evil

weigh L3 against 13

if scale is balanced, H1 is evil. if scale is unbalanced, L3 is evil.

if L1 + L2 + H1 is lighter than L3 + H2 + 13:

given the conditions above, L3, H1, and 13 are not evil. That leaves only L1, L2 or H2.

weigh L1 against L2

if scale is balanced, H2 is evil. If scale is unbalanced, the lighter stone is evil (since L1 and L2 were on the lighter side of the original scale)