Millie's shrinking formula was not chosen at random. Milli, micro, nano, and pico are prefixes (= a group of letters added before a word to modify its meaning) used in the measurement of smaller and smaller quantities. People are more familiar with the prefixes used in the measurements of larger and larger quantities such as kilo, mega, and giga.

Those prefixes are used in front of a unit of measurement such as meter (m) when measuring lenghts, gram (g) when measuring masses etc.

Here's what each prefix means:

Milli- (m): One thousandth (1/1000) of a unit. For example, 1 millimeter (mm) is equal to 0.001 meters (m). In other words, milli- means 1000 times smaller. That's the difference between the size of an elephant and that of a spider.

Micro- (μ): One millionth (1/1,000,000) of a unit. For example, 1 micrometer (μm) is equal to 0.000001 meters (m). In other words, micro- means one million times smaller. That's the difference between the size of an elephant and that of a red blood cell. The spider that seems so small compared to an elephant is 1000 times bigger than a red blood cell.

Nano- (n): One billionth (1/1,000,000,000) of a unit. For example, 1 nanometer (nm) is equal to 0.000000001 meters (m). In other words, nano- means one billion times smaller. That's the difference between the height of an elephant and the width of a DNA double helix. The diameter of a red blood cell that seems so small compared to a spider is 1000 times bigger than the width of a DNA double helix.

Pico- (p): One trillionth (1/1,000,000,000,000) of a unit. For example, 1 picometer (pm) is equal to 0.000000000001 meters (m). In other words, pico- means one trillion times smaller: 1000 small than of the width of DNA double helix. The sizes of atoms are measured in picometers.