From one cell to many
A multicellular organism also begins as a single cell.
A sperm and an ovum fertilise to form a zygote. That is one cell. It divides again and again to make thousands of cells, and those cells specialise for different jobs. So a multicellular organism starts in a unicellular state too.
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The number is not the point
Every cell made from the zygote descends from that same zygote, so they are genetically alike. Yet they do not all do the same thing. Some become nerve cells, some become muscle cells — and it is this specialisation that lets a multicellular organism do what no single cell can.
The next page looks at what those different kinds of cell actually are, in a plant and in a human being.