I love the way this project kept growing, from plotting catenary arches with hardware store chain, to building desktop models, to building 2-meter (and taller!) models. Inexpensive materials, straightforward process, memorable results. Without even having built one of the models, I will now remember that a catenary arch is an arch in equilibrium:
The shape of a hanging chain is called a catenary curve. Each link settles in to an equilibrium angle relative to its neighbors, so if you turn everything over, those angles give a plan for blocks which can rest on each other to make an arch in equilibrium. This suggests a fun construction: making giant arches from separate cardboard blocks.
And of course, I'm now trying to think of an excuse to build a giant arch...
(via Math Monday on Make)