One day, Cecrops, the first king of Athens, was looking for a patron diety to help Athens. He heard that maybe both Athena and Poseidon were interested in the job.
Since they both wanted the job, Cecrops decided the only way to have a fair choice was to have the people decide. A contest was held, and whoever brought the most valuable gift to the people would win he job.
Athena, you are the winner.
Poseidon used his trident to create a well in the middle of the town, but it wasn't fresh water. Poseidon made a salt water well, which the town had no use for.
THIS TOWN IS HEREBY CURSED!
On the other hand, Athena, stuck her spear in the ground, and grew an olive tree. The tree wasn't just an olive tree, it stood for peace and prosperity. Cecrops and the people liked this much more.
Cecrops and the people liked Athena's gift much more than they liked Poseidon's. Cecrops then made Athena his patron deity and named the city after her.
Poseidon didn't like the decison and he was very mad. He then set a curse on the City of Athens, cutting them short on water. This, the main reason Athens has water shortages today.