How can I track what users search?

I want to be able to track and record what users search for when they are on my site. I tried slim-stat plugin, but it only shows what page they ended up on in my site after a search. I want to see what they actually typed in to the search box.
For example, if they are searching for “monkey wrench” and come up with nothing, i want to know that my user is looking for a monkey wrench.

any ideas?