Synonyms: mystery, closed book, enigma, secret
Definition: something that baffles understanding and cannot be explained
Usage: how it got out is a mystery; it remains one of nature's secrets
Similar words: perplexity
Definition: trouble or confusion resulting from complexity
Synonyms: brain-teaser, riddle, enigma, conundrum
Definition: a difficult problem
Similar words: problem
Definition: a question raised for consideration or solution
Usage: our homework consisted of ten problems to solve