### Natural Clocks

Natural clocks are phenomena that keep time in some way as an incidental characteristic of a natural or every day process. Each such clock has limits as to its accuracy and the time range over which it functions (minutes, days, months, or years). Like any clock, natural clocks must satisfy certain conditions to be used:

- Irreversible process
- Constant or calculable rate
- Known initial condition
- Known final condition

Some natural clocks involve cyclical processes and are only irreversible (and therefore useful) within a single cycle. Other clocks are truly irreversible but reach an end point at which they stop keeping time (so they can only provide a minimum age for a past event if the process has ended). Students were asked to choose and describe a natural clock, and these papers are the result.