We love this book. It's an interactive pop-up. We started with the section related to circles and pi but ended up also learning about the Pythagorean theorem and ancient Egyptian mathematics (Egypt has been an ongoing interest for a little while now: there are Cleopatra and mummy movies, exhibits, and activities at our local museum right now; plus we all watched the Egyptian People's Revolution unfold recently, and Riley and I are still following that via an English language blogger/tweeter there and via the Global Voices website, even though it's no longer in the regular news much here).
Another side project we came across was making homemade balance scales. We used a hanger, two extra pie plates and the string that we already had out for measuring circumferences and diameters.
Comparing Bart and Kenny
Comparing Bioshock toys
Comparing Kenny & different numbers and size combinations of YooHoo friends toys
And, of course, what would Pi Day be without the other kind of pie!
Making pie crust
Veggie potpies & shepherd's pie