I learnd this one from the TopMod Forum. You should definitely check it out if you are interested in TopMod.
So this one is on how to create a Mobius String
We start with a Cube
Use the Cubical Extrude Mode to extrude 2 Faces of the Cube
Do it again, so that you get 2 Cubes at the end.
Select all Edges which are not on these 2 Cubes and delete them.
Now use the “Add Handle” Tool to connect the top of the Cubes. Take care that you get a nice Arc.
Delete the 2 Cubes
And add another handle on the bottom of the first
Press Space and typ: Doo Sabin and select the Doo Sabin Remeshing Mode
Go to Selection for Select Face Loop
Select 2 Loops…
… and use the Ring Modeling Thickness Mode