Your inability to understand this has led me to believe it is you who is not very bright. I will help you out here.
Currency- a medium of exchange used to buy and sell goods. Now, this needs to be commonly accepted.
Funny you would ask about the liquidity of paper money.......the liquidity of an asset is generally defined as how easily it can be converted into money.
So gold CAN be, and believe it or not has been, used as a currency. But as of this very moment, in this country, gold is NOT a currency. Gold is NOT generally accepted as a form of payment.
Personally, if i present you with a bill for $10,000, I will not accept $10,000 worth of gold as payment. I may accept a larger value of gold to make it worth my hassle, but my bill collectors accept Canadian dollars only.