I 'bumped into' my 6th grade math teacher on Facebook. When we reconnected, I commented on how one of her lessons is one of my favorite middle-school memories. We all had to come up with a numeric system and use it to display a calendar month. Mine was based on the Roman system, but used symbols instead of letters. A number of classmates chimed in with their own recollections; one did a base-31 system, so every day was unique. The teacher was surprised at how vividly we all remembered this one lesson -- 40 years later!