I have a great video that explains what it is.
Also if you are wondering about where to get the cardboard from, just look around you. Every school orders copy paper. One of my colleagues, Torri, shared with me today that you can make 25 rekenreks by using one Xerox copy paper box including the top, sized 7 inches by 4 inches. I have been playing with the size of the cardboard to make it just the right size for my kindergarten students. This is how I came up with 7 x 4 inches. I have been using the 9mm beads--pony beads are sometimes called.
I was planning on getting bigger beads to make a Teacher Size Rekenrek. There are some commercial websites that already have the rekenreks already made. I like it better when my students make the rekenreks by themselves. In the past, my students take better care of their "creations" and they are really proud of what they did.
Make sure to check YouTube for how other teachers are using Rekenreks in their classrooms.
Chao!
