Puzzle (noun)

1. A problem or game that tests one's ingenuity.
2. A game or toy consisting of small pieces that must be put together to form a whole.
3. Something that is difficult to understand or explain.

Origin:

From old french pousse, from pousser ("to push"), from latin pulsare ("to beat, to strike").

Examples:

1. The puzzle has 500 pieces.
2. The puzzle took him several hours to complete.
3. The crossword puzzle was challenging.
4. The puzzle was solved by the group.
5. The puzzle of human behavior is complex.