Invite students to create their own fraction, decimal, or percent practice pages for their favorite riddles. Remind them that for an activity with twelve problems, the answer to the riddle can be no more than twelve different letters of the alphabet. Have students write a letter below the answer for each problem. Then have them check their work and write an answer key on a separate sheet of paper. Collect the student-made activity pages, check them, then mix them up, and distribute randomly.