You may think, salt is just a simple cooking material that gives our food a little extra taste. However, salt is much more than that. Without salt our muscles (肌肉）would not move and our hearts would not beat.
Early humans got the salt they needed to keep alive from the animals they killed. But the improvement in farming led to a diet low in salt. So, humans needed to find other sources. Those who lived near the ocean or other natural sources were lucky while those who did not had to trade for salt. In fact, people used salt as a way of pay in many parts of the ancient world. The word＂salary" comes from the word "salt".
Salt also played an important part in population movement and world exploration (探险）. Explorers understood that if they could keep food fresh, they could travel longer distances. So they used salt to keep food while exploring the world.
Salt was so important that, according to food historians, it was traded pound-for-pound for gold. Today, people still use the expressions "to be worth one's salt" or "worth one's weight in salt". The expressions describe a person of value. A person might also be called "salt of the earth". It means that you can trust the person.
However, when you say "I think we should take what he said with a grain of salt", you mean you accept it but remain some doubt about its truth. Do you think rubbing salt in a wound will be useful? That would be painful and not cure the wound. Therefore, "to rub salt in a wound" means to make a bad situation worse.
