Riddle #5: Counterfeit Coin

Warning: Do not read other posters' comment unless you give up since it usually contains the answer. Clutchcity.net posters are very smart.

Another relatively simple one:

A foreign mathematician is captured by Sadam Hussein and were to be executed. Luckily, Sadam was in a good mood and he gave a chance to the guy to save his life if he could answer this riddle.

The guy was given nine coins. One of the nine coins is counterfeit. The counterfeit coin looks very much alike the others and can only be distinguished by weight - it's lighter than the others. The mathematician is given a chance to use a balance scale "only twice" to find the counterfeit coin. How will he do it?

Note: This has been answered by Invisible Fan

Previous Logic riddles

Riddle #1: Water Container http://bbs.clutchcity.net/php3/showthread.php?s=&threadid=52045
Answered by: drapg, SmeggySmeg

Riddle #2: The Wizard and 3 Caps http://bbs.clutchcity.net/php3/showthread.php?s=&threadid=52048
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Riddle #3: The Wizard and 3 Caps (Advanced) http://bbs.clutchcity.net/php3/showthread.php?s=&threadid=52050
Answered by: codell

Riddle #4: Average Speed http://bbs.clutchcity.net/php3/showthread.php?s=&threadid=52112
Answered by: Major, codell

 

Invisible Fan got it, although his explanation wording may be confusing to some

But I got what you mean and you're correct.