A simple, safe, and easy demonstration of the reaction of sodium peroxide with water is presented. The reaction is performed in a plastic bottle by adding water to sodium peroxide supported in a rubber "thimble"; oxygen is detected by the reignition of a glowing splint, while the generation of sodium hydroxide is indicated by phenolphthalein. The exothermicity of the reaction is evident from the melting of the rubber "thimble". The apparatus is easy to assemble, and small amounts of chemicals are required. The reaction is safely contained, and its physical and chemical products are readily observed.