Question mark
In the early 13th century, when the growth of communities of scholars (universities) in Paris and other major cities led to an expansion and streamlining of the book-production trade, punctuation was rationalized by assigning Alcuin's stroke-over-dot specifically to interrogatives; by this time the stroke was more sharply curved and can easily be recognized as the modern question mark.
Question (comics)
Early in his career, the Question used seemingly blank calling cards with a delayed chemical reaction that after a specified time caused a question mark to appear in a burst of gas. Other writing could be similarly treated to reveal itself at the pre-determined time.