Hilmar,

My boys will get a kick out of this. Are there any spaces (hollow gaps on the inside) when the problem is solved?

>To do this puzzle requires some manual work, but it's really worthwhile, I assure you.
>
>Get 20 spheres of styrofoam or something similar, and paste them together, into four pieces, like this:
>
>

>oooo
>
>
>oooo
>
>
>ooo
>ooo
>
>
>ooo
>ooo
>

>
>Each "o" represents a sphere.
>
>Now, try to fit the four pieces together into a pyramid (tetraeder). This is a quite tricky puzzle, although it only has four pieces!
>
>Hilmar.