Crypto currencies are not like real dollars. The IRS considers crypto currencies to be a form of property, which means that every crypto currency transaction, no matter how small, triggers a separate tax gain or loss. Retailers including some Subway sandwich shops are now accepting Bitcoin. But buyer beware. You'll have to report that transaction to the IRS!
"if you trade a piece of bitcoin to Subway in exchange for a sandwich, you have a tax realization event, meaning the IRS treats the transaction as if you sold the bitcoin for the sandwich."