Teachers, save “Pebbles” to assign it to your class.

Imagine that you're walking along the beach, a rather nice sandy beach with just a few small pebbles in little groups here and there. You start off by collecting just four pebbles and you place them on the sand in the form of a square. The area inside is of course just 1 square something, maybe 1 square metre, 1 square foot, 1 square finger ... whatever. (see attached picture) By adding another 2 pebbles in line you double the area to 2, like this: (see attached picture) The rule that's developing is that you keep the pebbles that are down already (not moving them to any new positions) and add as FEW pebbles as necessary to DOUBLE the PREVIOUS area, using RECTANGLES ONLY!