Have you ever noticed that some rounded stones are almost spherical, while others are kind
of flat like a bar of soap?
The spherical stones form that way because they never have the chance to settle into
a preferred orientation: they're either sloshing around in pot holes, or rolling along the
bottom of a river, or if they're small enough, getting blown around by the wind.
Since they have no preferred orientation, these stones get weathered the same amount
on all sides, and they'll end up looking the same from all sides as well.
That's spherical symmetry!
But for flat stones, even if they're a smooth disk perfect for skipping, there's obviously
a preferred, or special, orientation - the direction perpendicular to, or 'away from'
the flat sides.
Where did this preference come from?
And how do flat stones get flat?
Gravity.
It's kind of like the difference between dumplings and biscuits.
A dumpling is a lump of dough dropped in a pot of water, and gravity doesn't really affect
its orientation very much when it's rolling and bobbing about.
A biscuit, on the other hand, is a lump of dough pulled down by gravity onto a baking
sheet, so it tends to be flat and has a clear top and bottom.
And while the details are ultimately more complicated, when sedimentary rock forms,
it forms layers, or when stones too heavy to roll are worn down by abrasion at the bottom
of a river or lake, they tend to be worn a bit flat.
In both cases the preferred direction is dictated by gravity.
And gravity is also why we build buildings in layers, with a basement, floors and a roof
– if we were in outer space without a preferred direction from gravity, we'd probably just
live in blobs.