You all believe that Zeus fed Kronos a mixture of mustard and wine to make him puke up the rest of his children, am I right? Well, if that is what you believe, then you are wrong. The actual fact is that the Greeks were sexist bas... Oh sorry that's not what I meant to say. Zeus, in actuality, never lifted a single finger. While Hades and Poseidon we're in Kronos's stomach they forged armor by combining their magic. So basically Poseidon and Hades sliced open and cut up Kronos from the inside out.
When that was finished Poseidon took the throne, but was the sea god due to preferring the sea and his more easy going personality. Hades didn't want the throne and was more of the family-loving type guy so he took the underworld and gave Zeus the sky.