This is not a stool.
This is N° 019: a chair disguised as a stool. The chair is completely flat, until someone sits in it.
N° 019 is designed by Andreas Aas, a designer and architect who lives and works between Paris, France and Fredrikstad, Norway. Here's what he says about the concept:
When not in use, the chair occupies minimal space - both physically and mentally. It is only when used that the chair takes on the more strenuous shape of the human body.
With this in mind, the N° 019 must be the ultimate chair for Über-minimalist interiors.