Piast agrees to this, and in return the strangers bless his cellar so that it is always full of food. This magical cellar so impressed Piast's neighbors that they made him a prince, ousting Prince Popiel.
For his part, Popiel deserved ousting. He was a 9th century ruler of two proto-Polish tribes, the West Polans and the Goplins. A cruel man who cared only for his own pleasure, with a wife equally corrupt and power-hungry, he had twelve uncles poisoned at a feast because he heard they were going to depose him. He refused to give the bodies a proper burial. The townspeople, learning this , rebelled, and Popiel and wife fled to a tower. Miraculously, a mass of mice and rats that were feeding on the corpses of his uncles went to the tower, gnawing through the walls and devouring the corrupt couple.
Piast is unique in Polish annals, and its precise origin is unknown. It could come from Polish piasta, meaning "hub"; the could be what led to him being called a wheelwright. It could also mean he was a "hub" of activity/goods to support his people. Another theory relates the name to Polish piastun, "custodian." The theory there is that he was a steward or custodian of sorts for the ruler, who wound up replacing him. Something similar happened with the Carolingian Mayors of the Palace replacing the weakening Merovingians in Frankish Gaul.
Whatever the case, the first ruler of an independent Poland was Mieszko I (c.930 - 992), and he was a proud member of the Piast Dynasty, the first official one in a line that lasted until Casimir the Great, who died without a heir.
So ... what was the deal about the 7th birthday? Was that a special occasion? In pre-Christian Slavic tradition, it was time for a haircut, but I'll explain that next time.