If there are 14 people in a group, and each shakes hands with each other, there will be 91 handshakes. (Can you see why?)

91 = 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 + 11 + 12 + 13

(which makes it triangular)

and

91 = 7 * 13

(the middle and last numbers in the sum above)

Will this always happen for triangular numbers?

### Games & Puzzles

**Shannon Duncan**, a 6th grade math & science teacher, shares 4 Reasons to Promote Math Success through Games at the*MIND Research Institute*blog, illustrating her ideas with some of the games she has her students playing. I especially like the first point - making a mind-body connection.**John Golden**(@mathhombre) shares Angle of Coincidence at his blog,*Math Hombre*, about an angle identification game he's developing. Ask your students to playtest it and give him feedback! John also wrote about the start of the semester, and included a game called In or Out? that looks fun.**Jeff Trevaskis**shares a Multiplication Tic-Tac-Toe Game at his blog,*webmath***.****Carole Fullerton**shares Number Tile Puzzles at her blog,*Mathematical Thinking*.**Gray Antonick**interviewed Paul Salomon in the New York Times Numberplay column, about his Imbalance Puzzles, one of many puzzles and games featured in(my book, published in April!).**Playing with Math: Stories from Math Circles, Homeschoolers, and Passionate Teachers**

### Arithmetic

**Denise Gaskins**(@letsplaymath) shares an old favorite, 30+ Things To Do with a Hundred Chart, at her blog,*Let's Play Math*.**Brian Bushart**(@bstockus) shares Fraction Number Sense at his blog,*Teaching To the Beat of a Different Drummer*.**Lior Pachter**shares Unsolved math Problems and the Common Core at his blog,*Bits of DNA*. (Lior writes about computational biology. I found this post thanks to Andrew Knauft, at*LimSoup*.)

###
**Geometry **

**Stephen Cavadino**(@srcav) shares Parallelograms at his blog,*cavmaths*, on a student's creative way to find the area of a parallelogram.**Ioana I Pantiru**(@LThMathematics) shares Playing with Paper Folding at her blog,*Life Through a Mathematician's Eyes*, showing the steps of an origami construction. In her post, Maths Class Everywhere, she asks readers to take her survey of math classes around the world.**Curmudgeon**shares Circles on a Lattice, at their blog,*Math Arguments 180*. I wonder if this would make a good problem for a math circle...**Greg Blonder**, a professor of manufacturing and product design, shares Trisecting the Angle With a Straightedge, at*Plus Maths*.- There have been lots of posts in the past few months about classifications of pentagons (here's one), because a new (15th) type of pentagon that will tile the plane was recently found. Here's a good background post, from before the discovery, from the
*Mathematical Tourist*.

### It's All Connected

**Miss D**shares The Age of Ultron at her blog,*Miss D the Teacher*, about teaching a unit on artificial intelligence in a way that gets at the deep ideas and really gets students thinking, partly through connecting math, science, and art.**Henri Picciotto**(@hpicciotto) posts about Computer Programming and Math Education.- What is the distance to Mars? It changes depending where the two planets are in their orbits.
**John D. Cook**explains the math. **Michelle**shares Making Time for the Serendipitous at*The Rookery*.

### Ideas for Learning ...

**Kate Snow**(@katesmathhelp) shares How to Teach Your Kids to Read Math at her blog,*Kate's Homeschool Math Help*. I'm still trying to teach my college students how to read math, with some of the same tips.**Manan**(@shalock) shares Becoming Mathematically Fluent at his blog,*Math Misery.***Shecky**(@sheckyr) shares True Deep Beauty ... at his blog,*Math-Frolic*, about the how our understanding of math deepens.**Chris Rime**is making monthly math calendars (Algebra I, II, and Geometry), available as doc or pdf at his blog,*Partially Derivative*.

### ... And Teaching

**Tom Bennison**(@DrBennison) shares How to enjoy your NQT Year at his blog,*Mathematics and Coding*. [I had to look up NQT. It means newly qualified teacher, and in England and Wales, you are "inducted" in your NQT year, (generally) your first year of paid teaching.] I like his suggestion to make time for doing some math(s) yourself.

### Announcements

I'm going to the Joint Mathematics Meetings in January in Seattle. I'd love to connect with other bloggers who are going. There's a**math poetry reading**plus art exhibit on Thursday evening at 5:30. You can get all the details from JoAnne Growney's Intersections blog.