Zero conditionals are used to describe general truths and scientific facts where the condition and result occur simultaneously or are always true. They use the simple present tense in both the if-clause and main clause. Some examples of zero conditionals include: "If you heat ice, it melts"; "Plants die if they don't get enough water"; and "If you mix red and blue, you get purple." Zero conditionals are also used to give instructions using an imperative main clause such as "If Bill phones, tell him to meet me at the cinema."