Somebody among Rat, Snake and Rooster did a good deed. When questioned, Rat says, Snake did it. Snake says, I did not do it. Rooster says, I did not do it, either. Only one of those statements is true. Who did it?
We know that exactly one of Rat and Snake is telling the truth, since they are saying contradictory statements. This means that Rooster is lying, since only one statement is true, and thus Rooster is the one that did the good deed.
The key in these problems is to do casework based on the validity of certain statements, and to figure out whether or not a statements being true contradicts anything else you know to be true.