Place carrots into a clean canning jar, packing them in tight. Make sure to leave about 1 inch of headspace at the top.
Fill the jar with brine, making sure to cover the carrots completely. Weigh the carrots down to make sure they don't float (you can use a "fermenting weight").
Close the jar and let it sit at room temperature for 1-4 days. The longer it sits, the more the flavour will develop. Feel free to open and taste.