Expanding list in Sharepoint?

[MichaelSwanson]

I'm still new to Sharepoint, and am trying to make one of my options "expand." Specifically, I'd like to have a radio button for Stock and one for Customer. (Easy enough so far.) But if Customer is selected, the sub-fields Last Name and First Name will appear. Or remain grayed out unless Customer is selected.

I'm just trying to make the form foolproof for our users. (You know, the ones who don't care and will take any shortcut or ignore any instructions!)

Thanks for any help!