Not a silly question at all.
I will answer it with the definition for lawful money. First the definition that was repealed by Congress in 1994:
http://uscode.house.gov/uscode-cgi/f...20%20%20%20%20
And here is a more layman definition.
http://www.investorwords.com/2733/lawful_money.html
The courts say Congress simply declares that US notes shall be lawful money and that FRNs are redeemable in lawful money.
Regards,
David Merrill.