Three types of puzzles lurk:
1. Crosswords with themes, such as "PG-13," which has four 13-letter answers with the initials PG: "Euclid's field" = PLANE GEOMETRY.
2. Brainbuster crosswords, with tantalizing and tricky clues: "Present time"=CHRISTMAS.
3. Identical Twin crosswords, which are two puzzles with exactly the same grids and two clues for each answer, but you aren't told which clue goes with which answer: "One member of an explorer duo" followed by "and the other member" = LEWIS and CLARK.