A bombardier beetle produces and stores (separately) two chemical compounds which react with each other: hydroquinone and hydrogen peroxide.
When threatened, the beetle contracts muscles that force the two reactants through tubes into a mixing chamber containing water and enzymes. The reactants undergo a violent chemical reaction which gives off heat. This raises the temperature to near the boiling point of water.
The beetle swivels the tip of its abdomen around, and shoots a spray of boiling hot, poisonous chemicals at its attacker.