One of the really useful things about spreadsheets is that cells do not have to contain only data of a single type.  Cells can contain dates, numbers, text, and formulae (lists of instructions which tell Excel how to CALCULATE the contents of the cell).

 To put a formula into a cell, you have to begin with an equals sign.

This tells Excel that what follows is a formula.  If you leave this equals sign out, Excel thinks you are entering fixed text, and will otherwise ignore it.
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