You have 9 coins which all look identical but 1 of them weight less than the others. You have a two pan balance (see the picture) with which you can measure twice, and with this 2 measurements you have to determine which coin is the odd one out.
First you order the coins into piles of three, and take to piles to measure them. If they are equal the easy coin is in the 3rd pile so you keep that one, otherwise you keep the easier pile from the balance. You order 3 “piles” of 1 coin from the 3 coins left and take 2 of them to measure. Choosing the easy 1 is the same procedure as was with the first measurement.