This illustrates the geometry of the Möbius strip. We take a disc
with a blue line marked across it. We rotate the disc around the
$z$-axis. At the same time, the blue bar rotates within the disc
at half the speed. Thus, when the disc completes one turn around
the $z$-axis, the bar has only completed a half turn. In this
process, the blue bar sweeps out a Möbius strip.