[rudedog]

OK,

here goes

you have a total of 9 coins, you break them up into 3 groups of 3. you take 2 groups and weigh them against each other.
You have 2 possible solutions, they are equal or one is lighter then the other

If they are are equal then you take the last group and weigh 1 coin against 1 coin. if they are equal , your last remaining coin is the lightest.
or you will have a one coin lighter then the other

If one is lighter, you take that group and weigh 1 coin against 1 coin. if they are equal , your last remaining coin is the lightest.
or you will have a one coin lighter then the other