Abstract: A flask having a retractable body of approximately cylindrical plastic material formed by a succession of conical upper and lower sections connected at their apexes and in their troughs, an angle &agr; being formed between the conical upper section and a transverse plane and an angle &bgr; being formed between the conical lower section and a transverse plane, wherein the succession of conical sections alternate rigid/flexible, making it possible to obtain at least two stable extension positions, and a plurality of stable intermediate positions having axial symmetry, and wherein the radius of curvature of the troughs is less than or equal to about 0.2 millimeters.