On the vaporous surface of freshly brewed tea grows often a thin layer of scum. You will observe on the attached video that not only this layer appears to be crazed, but also that the mosaic formed by these cracks regenerates around every second in a new different mosaic, like a mosaic slideshow. This phenomenon may also be observed on very hot coffee. It looks like a chaotic phenomenon. The attached experience was made in a bowl with very hot plain water and green tea. The cracks suddenly propagate across the scum layer and produce a new mosaic. How to explain such a difference between the patterns ? What make the crack propagate ?