Fats oxidize when they are exposed to air. That's what makes them rancid. A butter keeper keeps butter fresh for up to 30 days by keeping the butter sealed from oxygen.
Press a cup of butter into the cup portion of the butter keeper. The cup will hold a cube.
Fill the little crock half full of fresh water.
To store the butter, turn the cup upside-down and insert it into the water. The rim of the cup will rest of the rim of the crock. The butter will be immersed in the water which will seal it from the air.
To serve, place the attractive stoneware cup on the table. Since the butter is at room temperature, it will be easy to spread.