Memory triggers

A week ago I told a friend about a project on of my students was doing. He said he wanted to offer his web site as a guinea pig. This week I saw the student and got him together with my friend. How did I remember to do that? I didn't write it down. I didn't make a special effort to remember. Yet when I saw the student, my friend's remark came to mind. What is in the brain that causes that to happen?