Age 7 to 11
You might want to make a box of the right size first and use some
cubes to try out your ideas.
Imagine you put cubes all along the base of the box and they were
joined together to make a cuboid. How high would you need to make
this cuboid so that you couldn't get another one the same size
inside the box?
Instead of imagining putting cubes to fill the base, how about
putting cubes up against a different face of the box? How far would
you need to extend this cuboid so that you couldn't get another one
in the same size?
But does the cuboid need to be touching all of one face?
In fact, does the cuboid need to be put in the box so that it is
parallel to the edges?