The line each ball moves along: the same in both boxes.
Fooled by depth
Ball and shadow
Watch the two balls. What does each one do?
On the left the ball rolls away across the floor; on the right it rises into the air. Yet both balls move along exactly the same line.
Why? A picture can’t show how far away a ball is, so your brain looks at its shadow, which lies on the floor. Shadow under the ball: it’s on the floor. Shadow left behind: it’s up in the air. Daniel Kersten and his team made this in 1997; this is our drawing.
Picture: Drawn by Science Playground, after Daniel Kersten