  Large circular ice blocks up to 80 m of diameter have been observed on frozen
river around the world. This rare event has been reported in a publication in
1993. This fascinating self-fashioned object slowly rotates at about 1$^o$ per
second. In this paper, we report a model experiment consisting in a 85 mm of
diameter ice disc at the surface of a thermalised pool. The rotation speed has
been found to increase with the bath temperature. Using particle image
velocimetry technique, we evidence the presence of a vortex below the ice
block. This vortex results from the descending flow of high density water at
4$^o$C. The vorticity of the vortex induces the rotation of the ice block. This
mechanism is generic of any vertical flow that generates a vortex which induces
the rotation of a floating object.
