Twelve shows the other Numberblocks how to arrange their blocks into columns and rows - and master the way of the rectangle.
Six and Eight see Twelve meditating at a place named Rectangle Retreat (name not mentioned in episode). They come to her. Twelve says hi to Six and Eight and allows them to watch and tell her if she's making a rectangle. When she arranges 3x4, Six and Eight display their factors and arrays; Six is 2x3 while Eight is 2x4.
They crash into a gong. Four comes by and asks them if she's okay. Next, Twelve teaches them how to turn like a rectangle. Four tries, but being a square, he thinks something is wrong because when he turns he is still 2 by 2! Twelve tells him that Four is a special kind of rectangle that is the same number of blocks wide as the number of blocks tall (2x2).
Four then crashes into Three and Two. He shows them his rectangle rays, but Three says she can't make wide rectangle shapes. Twelve has another method: she asks everyone to stand vertical; they are a tall, thin rectangle. They all start to fly around. Twelve doesn't mind, so she joins in.