Original: Thallium (I) oxide is the inorganic compound of thallium and oxygen with the formula Tl2O in which thallium is in its +1 oxidation state. It is black and produces a basic yellow solution of thallium (I) hydroxide (TlOH) when dissolved in water.
Simplified: It is made by heating thallium (I) hydroxide or thallium (I) carbonate in air.