At the end of yesterday's post of ten resources for high school and middle school math teachers I asked for suggestions for additional resources. This morning Colleen Young (whose blog is a must-read for math teachers) emailed me with the suggestion of NRICH.

NRICH is a provider of mathematics curricula and lesson plans covering everything from basic addition through advanced algebra and geometry. I initially reviewed it three years ago, but since then it has been overhauled for improved navigation. NRICH has sections for teachers and sections for the students. The sections have corresponding materials. For example, right now when you click on the page for secondary teachers you will see the featured activity is a time estimation lesson. A corresponding activity is presented to students when they click on "secondary students."

On NRICH you can find dozens of posters to download and print. Each of the posters displays a mathematics "trick" or challenge question. Teachers can download and print any of the posters in the collection. Each poster in the collection is linked to a problem page that contains notes for teachers using the posters.