←The standard Hering illusion (1861).
Placing the mouse over the image¹ causes the oblique lines to disappear, revealing the fact that the red bars were completely straight all the time.
[¹On a touch device, tap the image. To untap, tap on the icon top right.]
The picture on the right is a mess to look at. But if you place the mouse over it, you can see how beautifully it changes the apparent orientation of the red squares.
This is a dynamic variation of “Three in One” by Hans Kuiper which, in turn, is based on “Square of Three” by Reginald H. Neal (colour lithograph, 1964).
←Another variant, a combination of Hering’s and Zöllner’s illusion. Again, judge the straightness of the lines, then place the mouse over the image.