A list of seven, we learn, is a heptad. And history is rife with them: You got your seven sins, seven virtues, Seven Seals, seven ages of man, seven seas, seven continents, seven days of the week, seven digits in your phone number.
Why all the sevens? Because, as a formative psychology paper by George Miller found, people best recall things when they’re arranged in sets of seven (or so). And why’s that? Scientists are finding that’s the number our neurons really dig. So if you want to remember your list, stick to a seven.