Why were amphorae shaped like that?

[renfield1969]

Amphorae, the traditional clay pot of the ancient world. used for transporting and storing wine and water, usually had a tapered bottom. Why would you purposely make a pot to store a liquid that couldn’t stand up on it’s own?

-Renfield

[Melandry]

They don’t stand up on their own, but they do lay rather nicely in a curved ship’s hold… This cite even says those for home usage had flat bottoms.
More here:

http://www.bib-arch.org/bswb_AO/aojf00jars.html

The capacious holds of Roman cargo ships were filled in a herringbone pattern with hundreds, sometimes thousands, of heavy, thick-walled, six-gallon clay containers roughly 3 feet in height, called amphoras. Amphi means “on both sides” in Greek, and phero means “carry,” so an amphora was something carried by handles on both sides. This common shipping vessel also had a “third handle,” a long toe or foot, which enabled a stevedore to hoist the heavy container onto his shoulder or to roll it along the ground by rotating the two handles. The toe also supported the amphora when it was propped against a wall, rather like the end-pin of a cello or a double bass. In the holds of ships, one amphora’s toe fitted snugly between the handles of amphoras in the layer below, preventing the immense load from shifting. So the amphora shape was practical