hi all..... my first post on the forum....
hope it redeems my obvious greenhorn status

divide the balls into 3 lots of 4 each, called them ABC for convenience

Weigh A and B

They are the same weight

The ball is in lot C

Weigh A and C

That tells you if the ball is heavier or lighter

2 weighings over and you know that if the ball is lighter or heavier

You now only need to choose it from the 4 in that lot

Right so far?

If A and B were not the same weight

then weigh the heavier one (say A) against C

if A is still heavier, then obviously B and C are the same weight

so the ball is HEAVIER AND IN LOT A

If A is the same weight when you weight it against C,

then the ball is LIGHTER and is in LOT B

so this way also 2 weighings and you know

if the ball is heavier or lighter

and you know the four balls that contain the culprit

NOW YOU JUST NEED TOIDENTIFY THE ODD BALL

WITH ONE WEIGHING....

waiting to hear from y'all

cheerio