Okay, say you want to explain M-theory to someone whose only knowledge of strings is that they are 1) fundamental one-dimensional vibrating objects that have replaced the structureless elementary particles (pieces of atoms...), and that 2) their vibrations give rise to elementary particles with different properties, i.e., from matter to energy. Oh, and they know the analogy of a guitar string... They have a limited - only very basic - knowledge of physics. How would you explain M-theory to this person, have them stay alert[zz)], and have it make sense at their level of understanding? Oh, and I mean something more than "a theory that unites all five string theories, and supergravity, within a theoretical framework, but which isn't understood completely yet" ... I need something with a little more complexity, but without the need for them to run out and enroll in an advanced physics course, nor something that will turn them off to the whole subject. Think this is possible?