Only $$1$$ of the $$3$$ boys Abel, Ben and Charles can swim. Abel says, "I can swim." Ben says, "I cannot swim." Charles says, "Abel cannot swim." Only $$1$$ boy is telling the truth. Who can swim? 
Overseas Competition->Knowledge Point->Counting Modules->Permutations and Combinations->Combinations
Only $$1$$ of the $$3$$ boys can swim. Only $$1$$ of the $$3$$ boys is telling the truth!  Since Abel and Cain contradict each other, there must be one who is telling the truth!  If Abel is true, Ben is lying and Ben can swim. Then we have $$2$$ boys (Abel and Ben)  who can swim. Contradiction.  Hence, Cain is true. Abel is lying and so is Ben. Then, Ben can swim. Abel cannot swim and we are not sure if Cain can swim. 