Problems are quite challenging but ultimately solvable; the last major puzzle involving opening a Chinese safe is particularly ingenious.

A major puzzle is the force behind the rise.

A major puzzle is why so few textbooks and medical schools deal with the phenomenon.

So far, nothing they'd found presented major puzzles.

A major puzzle is why deaths from asthma have increased in many countries during a period when experts say treatment has improved significantly.

Obstfeld and Rogoff (2000) identifies this as one of the six major puzzles in international economics.

But the major puzzle was more subtle.

A major puzzle was also developing over the offworlders' reproductive behavior.

Content available with a paid subscription includes a database of words found in major published puzzles.

But then, the story of his life remains in almost every detail one of the major puzzles of ancient history.