i give you 9 tennis balls, all look identical in every way, 8 weigh exactly the same, one weighs an ounce or so more than the
other 8. i give you a weight scale (like the scales of justice)
and tell you that you can use the scale only twice and put as many balls as you like on either side but you only can use the scale twice after which you must tell me which ball weighs slightly more than the other 8
put three and threeon the scale. If they are the same, the other three contain the odd one.
a) if they are the same, then take two of the remaining three, one on each side, if the same, the last ball is the odd one.
b) if one side is heavier, take two of those balls, one on each side, and of the three, you know which is heavier.