RUNNING A FEVER

I came across this today when I had to send a mail to my client saying that one of my team members was absent since she is RUNNING A FEVER.

Why do they say running a fever...
You can run a dog, run a car, you can a have a running nose as well...
But how can you run a fever???

For sure, English is a very FUNNY language.

Update: Db suggested a few more
Raining cats and dogs...meaning heavy rains.
I knew sometimes small creatures like frogs and fish fall from the sky during rain, since they can easily get carried up during storms...but how can it rain cats and dogs!!!

Paint the town... meaning to go out and celebrate.
????

I got some good reasoning here.