Parents allowed their children to work for child laborers in such hazardous conditions because they think their child is illegible to do it, or they were to lazy to do it.
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After the Persian victories at Artemisium and Thermopylae, king Xerxes proceeded to Athens, which he captured in the last days of September 480. Meanwhile, the Greek navy, which had managed to get away from Artemisium, stayed on the isle of Salamis, opposite Athens. The presence of the enemy close to Phaleron, the Athenian harbor, created a strategic problem for the Persians: they could not use their port as easy as they wanted. And this was something they had to, because their army was proceeding to the Isthmus of Corinth, and it was imperative that the transport ships, brimful with food, could join the soldiers on the Isthmus. It was, therefore, imperative to expel the Greeks from Salamis.
Because, salt was needed to flavor food, preserve meat, used for medical purposes, and to keep the body healthy. But in West Africa, salt was rare. So, West Africans traded their gold for salt. Salt was so valuable, it was worth its weight in gold. 1 pound of salt = 1 pound of gold. How amazing.
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Labor's work hours were limited.