Odd coin out

There are 9 coins, all having similar look, appearance and other physical features except that only one coin among them is lighter than others. An ordinary balance with no weights is given to you and you have to find it out that 'odd coin' in minimum number of weighings. How will you find it out and how many number of weighings you will do so as to ensure minimum weighings?

Answer:

The odd coin can be found out in just 2 weighings. Label them as 1,2,3,4,5,6,7,8 and 9. Out of this, take any 6 coins, put 3 coins in the left pan of the balance and the other 3 coins in the right pan. If they balance, the odd coin is in the last 3 and can be found out in rest of the weighing. If they do not balance in the first weighings, still the odd coin is in the pan that looks up since it is lighter than others, and the same can be found out in the rest of the weighing.