Many processes that we have to deal with, biological and physical, do not follow a linear profile but much more of a curved profile.

A good example is the cooling down of an egg. When a warm egg is placed in a cold environment, the temperature of the egg will go down. This drop in temperature will not be linear, but curved. Initially, the drop will go very fast, but the more the temperature of the egg approaches the temperature of the environment, the slower the drop will be, until egg and air temperatures are identical.

The same holds for warming processes, drop in maternal antibodies after hatching or even the decrease in radiation after a nuclear accident.

These processes can be described with the so called "half times". The half time is the time necessary to go through half of the initial difference.
For instance, if the egg of 40oC is placed in air of 20oC, the first half time will be time needed to let the egg go from 40oC to 30oC, being halfway the initial temperature difference between 40oC en 20oC. The second half time will be the time needed to go from 30oC to 25oC, the third half time from 25oC to 22.5oC, and so on.

In principle, all half times are more or less equal in time. This means that if we know the length of the first half time, we know more or less the other ones as well. We normally consider the temperature to be constant after 5 half times. This means that if the first half time is for instance 30 minutes, we know that the egg will reach the temperature of the environment after 2.5 hours (5 x 30 minutes).

We can also use half times to determine what the environmental temperature must be to bring an egg at a certain temperature after a certain time.

The critical temperature for embryo development is 25oC. If we want to bring the egg from 40oC below 25oC in 30 minutes, an environmental temperature of 20oC is too high, when the half time is 30 minutes. With this temperature it will take two half times, being 2 x 30 minutes. In such a situation, either the environmental temperature must be decreased to 10oC (giving a temperture drop in one half time from 40oC to 25oC), or the half time has to be decreased to 15 minutes.

Half times in cooling are not fixed, but depend for largely on the air velocity over the eggs. More air velocity will increase the heat transfer from egg to air, and therefore will decrease the half times.

Half times for maternal antibodies or for nuclear radiation are much more fixed, and much more difficult to influence.