Aging is not typically measured by time in yeast, but rather by the number of divisions an individual cell completes before it dies. An individual cell is easy to follow from birth to death because yeast divides asymmetrically by budding off new daughters. Unlike their mothers, the daughters start from scratch, having the potential for a full life span. Thus, individual cells are mortal, while the yeast population is immortal. The probability that a cell will continue dividing decreases exponentially as a function of the number of completed divisions. Thus, mortality rate increases exponentially with age. However, it plateaus at older ages in similarity to what has been observed in other species. Yeasts undergo a variety of changes as they age, and some of these are clearly detrimental. In view of this, it is reasonable to speak of an aging process. In practical terms, yeast life span is measured by observing individual cells periodically under a microscope and removing buds with a micro-manipulator.