Take you class on a "field trip" to the younger grade classrooms when they are at recess. (Of course you would need to arrange this with the other teachers first!) Visit 4-5 classrooms and have them count all of the desks in each room. Assign a special helper to record your findings. Once back in your room, tell them that each desk has 5 text books in it. Their assignment is to figure out the total number of textbooks in the classrooms they visited.