Express the length of an object as a whole number of length units, by laying multiple copies of a shorter object (the length unit) end to end; understand that the length measurement of an object is the number of same-size length units that span it with no gaps or overlaps. Limit to contexts where the object being measured is spanned by a whole number of length units with no gaps or overlaps.
Express the length of an object in terms of unit lengths. Students should see that measurements are erroneous if there are gaps or overlaps. Measurements are also incorrect if the measurement doesn’t start at the beginning and stop at the end. Measure things that are very close, if not exactly, measurable by unit lengths. This way we can avoid talking about rounding up or down. That’s a topic for the future. Conclude by giving your students Cuisenaire rods from 1 to n and ask them how many different sized rectangles they can make. Don't forget the rectangle of height one!
TODO: Add an Amazon affiliate link for Cuisenaire rods.