This is an activity for upper elementary or middle school students. First, students solve puzzles, and then they make up puzzles. This video explains the basic idea, with specific page references to the worksheet:
Download the worksheet, which includes some teacher notes in the back.
Your students will also need some blank pyramids on which they can experiment.
Possible follow-up: I scripted a condensed version of this unit for a TED-Ed animated riddle. I recommend only showing this to your students after doing the activities I shared above, because the video jumps quickly to the final question, thus not allowing for a full exploration of the ideas. On the other hand, after they have done and discussed the worksheets, students probably would enjoy the animation, and appreciate the explanations that follow the pause.
This unit was inspired by Brad Fulton’s Pyramid Math (of which I only saw two pages.)
Many thanks to James Propp and James Tanton for valuable feedback.