A breathing device, comprising a mouthpiece forming a breathing channel, to form a connection between a first end and a second end of the mouthpiece； the first end being configured for a user breathing into the mouthpiece through a breathing opening； an at least partly flexible rebreathing air chamber attached to the second end of the mouthpiece, thereby being in fluid connection with the breathing channel； the rebreathing air chamber being formed by at least partly flexible wall section(s), the at least partly flexible rebreathing chamber having at a first wall section, being permeable to gas by a plurality of pores provided in said wall section and/or the mouth piece comprising one or more though going openings.
