The word “assume” occurs 48 times in What If?, and “assumption” appears four times. In science, assumptions are standard ways to set up thought problems. Keeping the starting assumptions simple helps delimit the problem and prevent it from becoming so unwieldy as to be unmeasurable and untestable. For example, in Chapter 51, “Lego Bridge,” the author says, “Let’s assume we’re building our bridge out of the most common LEGO piece—the 2x4 brick” (223). This simplifies things: There’s no need to recalculate the shape and mass of countless other brick types.

Sometimes, though, scientists, always searching for the most simple, elemental thing to study, oversimplify their assumptions, and the author makes fun of this in a footnote to Chapter 42, “Lost Immortals,” in which two humans search for each other on a planet. He quips, “Assuming a spherical immortal human in a vacuum...” (186).

Most of the measurements used are SI, the “Système International” of seven metric units: the second, the meter, the kilogram, the ampere, the Kelvin, the candela, and the mole. The US still uses older Imperial measures—especially the pound, foot, and yard—in commerce, but American scientists have mainly joined the international system. A meter is slightly longer than a yard, and a kilometer, or 1,000 meters, is about six-tenths of a mile; a kilogram, or 1,000 grams, is about 2.