Excel showing error message when typing “--do--”?

Question

Why Microsoft Excel Show an Error Message when typing "--do--"? It show an message "Your Formulae is incomplete".

Answer 1

Start your text with a single quote ' if you want to enter what Excel might think is a formula, because it contains numbers or is a mathematical expression.

The message box does say "If you are not trying to enter a formula, avoid using an equal sign, a minus sign, or precede it with a single quotation mark"

Answer 2

When you hit the first "-" it thinks you want to do a subtraction operation and tries to put a formula in the cell. If you first format the cell as text and then type that in you'll be fine.