Hundreds of Intriguing Math Problems
Stella’s Stunners is a well-organized, free collection of more than 700 open-ended, non-routine, brain-challenging math questions for middle- and high-school students, collected by former Oberlin, Ohio mathematics teacher Rudd Crawford. These would be perfect for the Peter Liljedahl “Thinking Classroom” structure, with students standing up in groups of three working on problems on erasable whiteboards (see Marshall Memos 976, 992, and 1014 for more on this approach).