R asked me this question at dinner time today. So, courtesy of google.com (!) we found out why. I knew this from primary school but wanted her to find out herself.
A bit of informal geography as R located the Hawaiian islands previously known as the Sandwich islands on the world map. Also, a bit of informal Islamic studies as we discussed why gambling is haram.
Interesting background to the sandwich: http://www.didyouwonder.com/why-is-a-sandwich-called-a-sandwich/
Looks like a good site for other ‘Why’ questions – Z’s favourite word at the moment!