The value of precious metals-like gold,silver, and copper-changes.When their value goes up,coins might be worth more in metal than their face value.In the past,people sold these coins as metal for more than they were supposed to be worth.Using less valuable metals makes sure the coins will stay in circulation,not get stored or melted down.

The value of precious metals-like gold,silver, and copper-changes.
诸如金、银、铜等有价贵重金属的（价值）变化。
When their value goes up,coins might be worth more in metal than their face value.
当它们的价值上升时，硬币的金属价值可能会超出它的面值。
In the past,people sold these coins as metal for more than they were supposed to be worth.
在过去，人们就曾经把硬币当作金属来卖掉以获取比推定值（面值）更多的利益。
Using less valuable metals makes sure the coins will stay in circulation,not get stored or melted down.
用廉价的金属来制造（硬币），可以保证硬币（稳定地）处于流通领域，而不是被用来储藏或熔炼。