The rookie card of James Brown is pretty well known in the hobby. Today, let's take a look at the back of the card instead:
Notice how there's a rather large are at the right with a trivia question, and that the answer can be seen when the area is rubbed with the edge of a coin. Here's a back where the answer is more visible:
Normally, the card that has been unscratched will sell for more than one that has been scratched, but this is a point that often gets overlooked by sellers who are content to just provide scans of the card's front. That can lead to some hard feelings when it gets sold to a collector who cares about condition and that collector flips it over.
Note to sellers: a scan of the back for all cards, not just the ones with scratch-off areas, might be helpful here. Collectors who are more condition-conscious will know beforehand whether the card has been scratched, and it'll help eliminate most of the returns that result. I know it's not many...but everything that staves off any headaches is a good thing in my opinion.
The scratch-off area has been with Topps football cards since before that. In fact, their football set in 1951 had a covered area on the back that could be scratched off:
The set's name -- Magic -- was a reflection of the fact that the colleges were obscured and that the owner was forced to look under the gray area to find out what it was. While many cards that have been scratched off look nice, the process of an 8-year old kid in 1951 using a penny or nickel to scratch the back didn't always lead to nice results. However, the scratch-off area doesn't seem to be as big a deal as one would think...unless the scratching left gouges in the card itself.
The process of having a trivia question with a scratch-off area was part of football cards (on and off) in both Topps and Philadelphia cards through 1970, but the gimmick has found its way into other sets as well:
For instance, here's the back of a 1963-'64 Topps hockey card of Stan Mikita that features a bilingual question...
And a 1964 Topps card of Hank Aaron that reveals a question about a home run champ...though not about Aaron or anybody on his team.