Islands or Continents

I have been helping out an the elementary school where my wife teaches. Last week I had a smart little 4th grader ask what the difference is between an island and a continent. Of course I answered the continent was bigger. So, they ask why is Australia a continent and Greenland an island? I really had to think... Most of the geography answers really didn't help. But, the geology answers I came across seem to make sense, although the students did get that iced over look in their eyes as I rambled on about continental crust vs. oceanic crust, plates and geologically stable cratons. However, it was when I started to discuss the ideas of island arc accretion that I realized I had lost them. Perhaps next year.