Alice, with her ETHUSD piggy, can put it up for auction, where anyone with access to the Ethereum network can purchase it. Let's say Alice is selling protection, or insurance, against the Ethereum price in US Dollars moving lower, i.e. ETHUSD is $100 and the piggy will payout if the price goes below $80. Carol is looking for exactly this protection, to insure her ETH holdings. After Alice places her piggy on auction, Carol can buy it. And if the price of ETHUSD goes below $80, Carol can break open her piggy, while the SmartPiggies smart contract automagically distributes the winnings to Carol and the remaining funds to Alice. Alice is the creator or writer of the piggy, and Carol is the holder.