looking for a behavior

I want to find a behavior that when you run your mouse over a link, a little
pop-up window will appear giving a brief description of what the link has as
its contents. I'm having trouble finding a behavior to do this. I tried "sho
pop-up menu" from behaviors, but after I put a couple of these on this on this
one page, they quit working. Now if I try to add them, they wont work. Is
there someone who can help?