"Let two bulls be given to us ... And you call upon the name of your god, and I will call upon the name of the Lord, and the God who answers by fire, he is God." (1 Kings 18:23-24, English Standard Version)
C.G. PfannschmidtSo, basically, both teams have to build an altar and offer a sacrifice, then wait for their respective deities to light 'em up -- fairly standard my-god-can-beat-up-your-god stuff. Naturally, Baal fails to show up to the party, and his prophets start getting exasperated, so they proceed to do whatever they can think of to elicit a response: they limp around, they cut themselves open with spears, but Baal does nothing.
And then Elijah gets bored and unleashes the sarcasm:
"And at noon Elijah mocked them, saying, 'Cry aloud, for he is a god. Either he is musing, or he is relieving himself, or he is on a journey, or perhaps he is asleep and must be awakened.'" (1 Kings 18:27, English Standard Version)
Some versions interpret the "relieving himself" part as simply "he's busy" -- but no, it's "I bet your god hasn't shown up because he's taking a dump." The original Hebrew word is sig, which can mean "expulsion," defecation," or "bowel movement." Yep, Elijah is being about 900 percent more offensive toward those dudes' deity than Monty Python was of Jesus in Life of Brian.
Oh, and in case you're wondering, Elijah then proceeds to build his own altar and drench it in 12 buckets of water, and then God lights it up in an instant.