number sense in secondary education

Usually, when I think of building number sense, I think about primary math education.  I know, though, number sense is something we all must work on all the time!  I'm curating ideas for secondary math and building number sense.

1. Knowing how precisely a high school freshman can estimate the number of objects in a group gives you a good idea of how well he has done in math as far back as kindergarten, researchers at Johns Hopkins University found.  This Science Daily article is quite interesting.


2. This is a go-to book for ideas: Building Powerful Numeracy for Middle and High School Students, written by Harris and published by Heinemann.  


3. This site says it's a community for number string design!


4. The state of Texas has a well-developed academic competition developed by the University of Texas - one of which is number sense.  Practice tests are available online here.
   


5. The San Francisco Unified School District Math Department has put together a page full of resources on number talks. Check it out!


6. Fawn Nguyen put together this set of 90-number talks for middle schoolers — enough for a full school year if you do this routine every other day.

What tools, routines, or materials do you use to build number sense in secondary students? How often do you work on number sense? How important is it in your math program?