If your aerator has broken parts or a hardened washer, you are going to need a new one. But don't just drop everything and walk into the DIY store empty-handed. Take your ex-aerator with you. This will make your life much, much easier. Home improvement stores like Home Depot have a section with replacement aerators and a template that you can use to get the correct product. Before you go, it will help you to know that aerators come in male (threaded on the outside), female (threaded on the inside) and dual thread (both). You'll also be happy to know that the chances of finding the right aerator are 9-1 in your favor.