My 4th grade worked on optical illusions. I love teaching this 'magic' to kids. They love the final outcome of the illusions we created. I have 5 4th grade sections so I did a different lesson with each group. They all have the work hanging in the hall so each class was exposed to other options of op art.
This project was inspired by this pin. I'm not sure who it belongs too, but here is the link on Pinterest. I also used the instructions from We Heart Art. The kids made the art work with sharpies and I took their picture standing on a step stool. They cut them out and glued them on the falling tunnel.