Every living thing has at least one cell. 每个生物至少有一个细胞。

　　Then, what is a cell? 那么，什么是细胞呢？

　　Every cell works differently, but most cells work to maintain life. 每个细胞的功能都不相同，但大多数的细胞都有维持生命的作用。

　　Therefore, a cell is often called the building block of life. 因此，细胞常被称作是构成生命的基本单元。

　　A small house needs a small number of bricks, a big house needs many bricks. 小房子需要少量的砖块，大房子则需要很多砖块。

　　Similarly, small organisms need only a few cells. 同样，小型生物仅需要几个细胞。

　　For example, most bacteria have only one cell. 例如，大多数细菌仅有一个细胞。

　　But big organisms like animals or plants need a lot of cells. 而像动植物这样的大型生物则需要大量的细胞。

　　How many cells do people have? 人有多少个细胞呢？

　　They have about 100 trillion cells! 人约有100万亿个细胞！