What did you make today?

[Clodfobble]

That is supposed to be a cutaway view--but I realize now that from a 3-dimensional perspective, the red and pink ought to be flipped, so darker is farther away from you. You can't tell from the picture, but one side is slanted higher so when you drop the marble in it rolls quickly down to the other end. What he's envisioning is vertical, like this, but much more complicated, and with marbles instead of cars.