They require a Social Security Administration-issued card. Which I had, once, somewhere, but can't find anywhere. No, it's not in my wallet--the SSA tells you not to carry it with you. It should be with my birth certificate/passport/car title/etc, but it's not. I've been through every box of paper in my apartment, and it's nowhere. I think the last time I saw it was 2003, or four moves ago. ("How did you get your last job?" "They never asked for it--I filled in the the I-9 form and faxed it to them.")
I can get a replacement...sometime, eventually, cards are mailed out of Baltimore in the order the applications are received. The E-Verifying has to be done before I can start work.
I wonder how much it would cost to get the guys who sell fake documents to illegal aliens to make me up one with my real number on it...hell, how do you find these people?
UPDATE: USCIS puts the I-9 directions online, where I can see it.