The magician walks onstage carrying an ordinary briefcase that he puts down on the table.  When he opens it up he removes a real bowling ball or basket ball or other large item that cannot possibly fit in the briefcase.  After the production the case can be shown all around. 

This is one of the funniest props for the comedy magician.  Can also successfully be used by a clown or M.C. This is an improved version of the effect where you can show both the inside and outside of the briefcase at the end of the effect.