Christina Tondevold is a Math Consultant and author of the book "Mastering the MathRack". http://www.mathematicallyminded.com/ "With the MathRack, children can model and solve problems the same as they would with other manipulatives, but the build-in structure of the MathRack allows children the flexibility to develop more advanced strategies. I've seen the number sense that children develop as they work with the MathRack; it is powerful and helps them as they progress through higher levels in mathematics."