Window Trap

If you want to ventilate your room all the whole flat is efficient to to keep the window open as far as possible.
One of the challenges to master was to create a thing which can be glued with double sided tape to the window sill so you can remove all the time without any marks.

So you can open the window turn the clamp upwards and you're done - if you want to close the window just lay the clamp flat on the sill.

(If you swap the holders you can flip the clamp in the other direction)