It’s been three days now that a very important operation is going on: invisible sand transportation!
The loading takes place at the right side of the coach, where a green (and perfectly visible) truck is being loaded with cargo. Then, it pulls out (very carefully, not to loose a single grain), and goes to a building site (invisible).
Once it reaches the left side, it stops, lowers the loading platform and unloads the sand onto an invisible heap.

There’s no more left! (A hand gesture.)
All the sand is unloaded, and the truck is empty!
Look mommy, empty!
Yes, Leo, empty!
The truck goes back and rushes to the loading site…

And this goes on.

Imagination knows no limits…
Still from "Neverending story"