In the novel I just finished, the protaganist's mother is declining into dementia. Every time she goes for a check-up, the doctor asks her various things to establish how far gone she is. The usual: her name, the date, the name of the President, and then, invariably, she is asked to count backwards from a hundred by sevens.
By sevens? For the record, in case this skill is ever the thing that stands between me and the loony bin, I just want to state that I can't count backwards by sevens. Not now and probably not when I'm on the brink of senility either.