Life is never for certain there are so many unpredictable things that can happen like at one moment you are turning onto the street and you get into a car accident you didn't plan for that to happen. Anything can happen on a day to day basis that's why something can go wrong and all your plans won't work anymore.
Units and prefixes are created and unit definitions are modified through international agreement as the technology of measurement progresses and the precision of measurements improves.
You could flip a coin twice. The result of the first toss chooses the morning activity, and the second toss determines the afternoon choice. There are only 4 different day-schedules available.
