I would use an analogy. RAM is like play-dough, as ROM is like a building block. Play-dough can be repeatedly reshaped to resemble a different object, just as RAM can be rewritten over and over using different data. Building blocks are solid and cannot easily be changed. In fact, changing them destroys the whole object. Alternatively, you could just use a single simile. Memory is like play-dough. It is RAM when it can be reshaped and continually manipulated to resemble any form (data), but if it's left to dry out it solidifies and can no longer be (easily) manipulated (At which point I guess you could say it has become directly comparable to ROM).