When comparing two values, such as mass or temperature, we often don't care about the absolute
values rather than a comparison. So, for example, we might say that Jack is twice as tall as
Jill. Or, in scientific terms, we would say that Jack is a factor of two taller than Jill.
Notice that their heights in inches or meters is not mentioned.
We can also reverse this and ask if Jack is 100 kg in mass and Jill is 50 kg, how big is Jack
compared to Jill. This is expressed as a ratio:
So we say that Jack has twice the mass of Jill, or that Jack is a factor of 2 greater than
Jill. Likewise, we could say that Jill is a factor of 2 *less* than Jack.