Hwo do i keep a ice cube from melting?

i have this project in school and i have to build a box that can hold a ice cube and the ice cube can not melt for 6 hours any ideas? oh and it can not be any thing that u can freeze even snow heres my idea i need u to see if this is a good idea: a box with tin fowl and other ma trials and a Little box inside the other box that has the ice cube in it and satire foem (idk how to spell it) and a fan inside the box is that a good idea i ned help fast any internent things will help i need answers yahoo people answers plzz :)