New Words：

planet ['plænit] n. 行星；地球（尤指环境）
what planet is sb. on? 指某人的想法完全不切实际

Sample Sentences：

The planets move around the sun.
Does life exist on other planets?
They claimed to have discovered a new planet.
Mars is the fourth planet in order from the sun.
He thinks being a father is easy. What planet is he on?

exist [ig'zist] v. 存在；生存
claim [kleɪm] v.声称；宣称

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