Lines at the squares’ sides: no square ever crosses them.
Tricks of motion
Squares that march
Which way are the two rows of squares marching?
Neither way. The top row seems to march right and the bottom row left, but not one square moves.
Why? Only the colors change: each square fades lighter or darker while its dark and light edges swap sides. Your eye’s motion sensors read those changes the same way they read a step. The Japanese artist jagarikin makes animations with this trick.
Picture: Drawn by Science Playground, after jagarikin · Wikipedia(opens in a new tab)