We present an extraordinary variation of the X-ray spectrum of the obscured AGN in NGC 1365, which was observed by Chandra to change from Compton-thin to Compton-thick and back to Compton-thin in four days. This fast variation imply a a size of ∼ 10 14 cm for the emitting region, and an extremely compact size (∼ 10 16 cm) of the clumpy circumnuclear absorber.