How did Medusa get snakes for hair?

Usually the story is that Medusa was an Athena priestess, who once made love to Poseidon in Athena's temple. In revenge Athena turned Medusa into something as ugly as possible.

Another Answer:

Because she was making love in a temple that belonged to Athena and in rage Athena turned Medusa's hair into snakes, or Medusa bragged about her hair and Hera, out of jealousy, turned her hair into snakes. Whatever makes sense to you.

Yet Another Answer:

Medusa once informed Athena that Snakes were more Fair(more beautiful) than Athena herself in order to spite her. In a fit of rage Athena damned Medusa to become that which she held so high.