My students were working on creating word problems with expressions using variables. They created a book to show what they know. On the inside, they made a QR code that had their problem and then on the back side was the answer in QR code form. After the kids made their books, we set them around the room and they went around to scan their friends problems to see if they got the same answers. There was great debate when a few came up with different answers. They had to prove why they thought they were right.