Q: when is a space not a space?

A: when it’s a non-breaking space.

Q: What’s a non-breaking space?

A: A non-breaking space is used when you want the text on both sides of the space to stay on the same line of text.

Q: Why should I care?

A: Mostly you don't need to care. The app you are writing in will take care of line breaks and wrap your text as you type.

It is a problem though if, for example, you leave a space between a currency abbreviation and the number, or write large numbers with a space rather than a comma. You do not want to have something like this: The price is EUR
10.99 or this: £750
000

Q: What can I do to prevent that kind of thing?

A: You can type a non-breaking space instead of a standard space. You do this in Word with the key combination Shift+Ctrl+Spacebar. Now, the whole number is treated like a single word and will appear together on a single line.

Bonus tip

You might run into a similar problem sometimes where your writing app breaks a line at a hyphen. Mostly that will be fine but you might want, for example, a word like co-ordinate always to be on one line. There is, you've guessed it, a non-breaking hyphen that works just like a non-breaking space. You type one by Shift+Ctrl+Hyphen.