Clues that end with a question mark serve to alert solvers to the presence of wordplay in those clues. These clues stretch reality a bit, and require solvers to think creatively. That’s part of the fun of solving a later-week puzzle.

For example, in the Thursday, Aug. 8 puzzle by Timothy Polin, the clue for 9A was “Motion pictures?” It’s easy enough to think that the clue refers to the synonym for movies, but then the question mark would not be necessary. Instead, solvers are asked to think about other kinds of pictures that suggest motion.

The answer was OP ART, short for “optical art,” the abstract genre from the 1960s that was based on geometric shapes with strong contrast that gave the perception of movement in the piece. The genre grew from the Neoimpressionism, Cubism, Futurism, Constructivism and Dada movements. Some op artists, such as Julio Le Parc, made pieces that did move, but many of them used the physiology and psychology of perception to create patterns that fooled the eye.