[PopAlongKid]

(gets 1978 copy of The C Programming Language off the shelf)

   if (year % 4 == 0 && year % 100 != 0 || year % 400 == 0)
      [it's a leap year]
   else
      [it's not]

I remember this because it is where I first learned the extra rules, beyond just every 4th year.