## Length Unit Relater and Repeater

##### Sub Trajectory: Measurement

Measures by repeating (iterating) a single unit and understands the need for equal-length unit. Relates the size and number of units (inverse relationship). Can add two lengths to obtain the length of a whole. Often can use rulers with minimal guidance in straightforward situations.

### Other Examples:

• Child repeatedly lays down a single inch unit to accurately measure an object.
• Child says, "If you measure with centimeters instead of inches, you'll need more of them, because each one is smaller."
• Child says, "This is 5 long and this one is 3 long, so they are 8 long together."
• Child measures a book's length accurately with a ruler.

Activities help children learn the two components of this level. The "relater" tasks ask children to predict the results of, and also to engage in, measuring using different units. Discussions are important in helping children internalize the inverse relationship (the longer the unit, the less of them are used). The "repeater" tasks ask children to iterate (repeatedly place and move) just one unit.  Children often are not worried about accuracy as we might (e.g. they may slide it along an object without concern about placing the unit so it starts exactly where it "ended" on the previous interaction), so the best activities are self-checking (e.g., if you measure correctly, you "find the one that fits perfectly").  Having a mixed-up puppet make mistakes and asking the children to help him can be fun and generate good discussion and solid concepts.