Enter a clue or pattern or both:
The ClueCauses of head-scratching (7 letter answer)
NotesThe answer ENIGMAS is seen occasionally, appearing about once every 650 puzzles.
Hard nuts to crack
W.W. II code machines
They're hard to figure out
DefinitionENIGMA as a noun:
1. (mystery, enigma, secret, closed book) = something that baffles understanding and cannot be explained; "how it got out is a mystery"; "it remains one of nature's secrets"
2. (riddle, conundrum, enigma, brain-teaser) = a difficult problem