There's a sign on a fence at a local theme park that says 'Six foot man eating chicken'. Next to it is a knothole that you're supposed to look through. When you peer in, you see a mannequin of a man eating what looks like fried chicken. The mannequin is presumably six feet tall.
The joke is you're expecting to see a chicken that is six feet tall and eats men. When you see something else, you laugh and say something like 'oh, you sure fooled me.' Interestingly, the park staff was correct in their use of the English language. It is the readers who are misled.
This is why writers often use hyphens for temporary compounds. Had the staff of the park actually wanted to say it was a chicken that eats men, they should have written 'Man-eating chicken.' Of course there's no such thing, and they would never have occasion to write that.
Another example is the difference between 'Man eating shark' and 'Man-eating shark.' The presence of the hyphen provides clarity, the shark eats men. Without the hyphen, the reader could assume it is a man eating shark meat, which is plausible yet different from what the writer may have intended.
The rule is when a temporary compound is used as an adjective before a noun, it's a good idea to include a hyphen to show the reader the two words are intended as an adjective. An example of a temporary compound is putting the words 'fast' and 'moving' together to get 'fast-moving'. This phrase when put in front of 'van' says 'fast-moving van'. Otherwise, the reader could mistake what the writer intended to mean a moving van that moves fast.
I'd like to say due to my superior knowledge of the English language I instinctively know about temporary compounds and hyphens. The truth is, up to a year ago I had no clue when to use hyphens. Then someone critiqued an article I wrote and pointed it out. It was very embarrassing to me. My hope is that by reading this post someone else out there can avoid this pitfall.