For this lesson, I tell the students we are going to be learning about standard units of measure. For homework they are to bring in an object from home.
During the math lesson I assign about five things in the classroom to measure. Their desk, my desk, the bookcase, the blackboard, my file cabinet, etc. Before the lesson started, I use my object to measure the width of the doorway (I let them see me do this)
They record how many of their objects wide or tall it was. While they are measuring, I am walking around to assist. Then we come back together.
I write on the board how wide the doorway was
Doorway: 8 units
I ask them how wide their objects were and record a few answers
Teacher desk: 10 units, 3 units, 34 units.
I ask them if the desk could fit through the door if the door is 8 units and the desk is 10 units (no way!).
This gets us to talking about why it's important that we are all using the same units to measure. If my cabinet maker is leaving space for my dishwasher, he and the dishwasher maker need to have the same measurements so it fits properly. If the cabinet maker is using his cellphone to measure the space, and the dishwasher maker is using his guitar to measure the washer, we are going to have a problem.