“How can we model how long something is expected to survive?”
One of the most fundamental laws of physics is that the total amount of entropy can only increase. Consequently, everything in the physical domain will wear out over time and eventually no longer function. Since the survival of many projects and people are responsibilities of many engineers and medical practitioners, these professions are interested in how long things are expected to last. This can be accomplished using a Reliability Function, (also known as a survival function in medicine), which models the statistical lifetime of an object of interest as a cumulative distribution function. These can play out in any number of ways, but the starting point must always be equal to 100% and the endpoint zero.
Image credit upload.wikimedia.org