Homework Help: The smallest coefficient of friction.

I just started with my Grade 12 Physics (Univeristy Level), and I would like some help (not necessarily an answer) but an explanation. Here it is:

A small box is resting on a larger box, which in turn sits on a table. When a horizontal force is applied to the larger box, both boxes accelerate together. The small box does not slip on the larger box.

If the acceleration of the pair of boxes has a magnitude of 2.5m/s/s, determine the smallest coefficient of friction between boxes that will prevent slippage.

I have drawn a free-body-diagram, and I have determined that the force of friction is what makes the small box move with the larger box. I do not have a teacher, other than for one period of the day for this course, so I will either have to wait to talk to him, or someone could pitch me something useful. Thanks.