During the period when children use the rods only for building, they are quick to show an appreciation for the manner in which colour is used to denote length, and how colour identifies rods that can be placed end to end to make up the lengths of other rods.
That the children have noted the physical properties of the material is confirmed when they answer questions about the rods, with accuracy and with their eyes closed.
We can work on this knowledge, once acquired, to provide the further vocabulary enabling them to inform us about their activities in the precise terms customarily used in mathematics.