How Big is a Foot by Rolf Myller and Susan McCrath

This is a great book to use when first starting a measurement unit. 

Read the book to the class and discuss the consequences of what life would be like without standard units of measure.

After you read the book, measure in inches the length of each child's foot.  Discuss how the results would differ on the size of the bed constructed if the smallest foot or the largest foot in the class was used.