(This is a stab in the dark, but let me know if you try it!)
I'm tempted to say "Read Flatland by Edwin Abbott", but that will take some extra explaining.
Have them draw an outline of their hand on a piece of paper, cut the drawing out of the paper.
Now, ask them to compare the the width of their hand to the "width" in the drawing. (the axis from their thumb to their pinky). Then have them compare the "height" of their hand to the one in the drawing (wrist to the tip of their middle finger).
Hopefully, they will notice that they are about the same.
Now, what about the "thickness" of their hand? Is it the same as the paper? Probably (hopefully) not.
How would we make it so that the "thickness" was the same?
Well, one way would be to use a rolling pin and squish their hand as flat as the paper.
Spheres work the same way, when you look at them from one direction, they have a width and a height, but when you look at them from another angle, they have a thickness, like a person's hand.
You might consider repeating by outlining the hand clenched in a fist, or get a ball of dough and use a rolling pin. Look at the mess it will make, the dough must go somewhere when you reduce the thickness (a dryer dough that will crack will demonstrate this better).