Unfortunately, the business school I linked to changed or dropped their link to their page that explained the UPC code.  Nevertheless, I did save a copy of their diagram which clearly shows the line patterns for each number.

Compare the pattern for the number "6" (even) with the patterns for the right and left guard or acceleration bars, and the centre acceleration bars, found at the top.  Note that only the number "6" matches any of the 30 following  patterns!

DO NOT simply trust what I am saying here.  Go out there and LOOK at A LOT of bar codes on various products in the stores until you convince yourself that 666 is on EVERY bar code!