An anonymous reader writes: As an engineer who studied in Asia for most of my life, my first exposure to real mathematics was when I arrived at graduate school in the United States. While I did take and enjoy some basic courses in mathematics (like real and functional analysis, measure theory and probability), I had a tough time because I found myself having to train myself in making rigorous proofs/arguments, compared to the engineering approach. I also found that training invaluable in helping me in other aspects of my life (including my engineering job). Now that I am back in my home country with children of my own, I see that the curriculum and approach in mathematics hasn't really changed. Rather than getting them used to thinking and making concrete arguments, they are taught formulae and most of their homework and exams focus on number crunching. So I'd like to ask slashdotters: What books/activities would you recommend for students in the 5th-12th grades (or even earlier) that might get them to appreciate rigor and critical thinking?