Rekenreks are great tools that provide a visual representation of the math problems. The students learn to immediately see sets of 5s and 10s within the numbers they are working with. They can then branch out to see other patterns as well, i.e. 2s, 3s, etc. I made mine out of 3" x 6" craft boards (pkgs of 6), bracelet string, and pony beads.
