Help on Enhancing Search Form with jQuery to Dynamically Show Certain Sub Options

I want to know how to enhance a search web form with jQuery to dynamically show certain sub choices once the parent field is filled out. For example… take Amazon’s website and how they have a form to Find The Right Ink Cartridge. You select your printer manufacturer/brand, then the next form field dynamically asks for more info related to your first selection:

Amazon.com Ink & Toner Finder: Ink cartridges, laser toner cartridges, printer ink cartridges and laser printer toner compatible with your printer.