Fun with SumBlox

We have been having fun with our SumBlox, which recently arrived.  The number blocks are cleverly designed so that the height of each is proportional to its value.

Here we have the ten block

and here we have multiple mathematical ways to achieve the same height as the ten block:  five + five, three + seven, and nine + one.

So far, playing with SumBlox seems like a fun way to build number sense and explore basic properties of addition like equivalence and commutativity.  But there does seem to be one problem:  I think they got the height of this block wrong!