The optical illusion here is that, due to the angles of the lines, the midline (above middle, really, going from left to right) looks like it is tilted down to the right, but is in reality level.
This idea came from gavtrain.com along with the printable backgrounds.
A fun thing to practice, especially during this shelter-in-place time.
I kept having to use a small bottle brush to try to get the bubbles off the glass walls! What you are seeing on the "table" is actually a texture of our cooktop. ;)