On one of the many adventures of Thor, he travels to the land of the Giants. Where he ends up in several contests that he thinks he should be able to easily win.
One being the giants drinking horn. The giant said Thor wasn’t a god if he could not drink the contents in three gulps. Thor takes one huge gulp, two, three. The horn somehow still seems full. He’s crushed that he could have lost at a drinking game!
What Thor didn’t know is that the giant had tricked him. What he was seeing was a drinking horn of mead. What he was really drinking was the contents of the entire ocean!
The story says that Thor drank so much of the ocean, much more than the giant thought he would, that the level of the ocean dropped significantly. And the reason we now have tides was because of the Drinking Horn.