Optical illusion is a topic that can be taught cross-curricular, in biology in which way the human brain is cheated, in art lessons how to create own illusions ( also in 3d) and in mathematics talking about the geometry behind this illusions. Students like to create it witrh GeoGebra very much. It was a challenge to create figures that can be dragged. 

• Optical illusions with Geogebra (Pestalozzi Gymnasium Unna)

More work in a GeoGebra book