cup
Explanation
In this comic, a woman asks someone to hand her "five hundred sextillion terabytes of flour." The man responds with exasperation: "Say 'cup.' Just say 'a cup' and I'll give you a cup." The woman huffily declares that "mass-energy equivalence and the fact that information is physical are just pointless discoveries to you."
The joke is about a person who insists on measuring cooking ingredients using physics-based units instead of practical kitchen measurements. She has converted a cup of flour into its equivalent mass expressed as information content -- using the Landauer principle (which states that information has a minimum physical energy cost) combined with Einstein's mass-energy equivalence (E=mc^2) to express the flour's mass in terabytes.
The caption at the bottom reads: "So far, no takers for my Einstein-Landauer culinary units." This is a classic SMBC gag about an overly enthusiastic scientist who applies rigorous physics to absurdly inappropriate everyday contexts. The humor comes from the collision between the technically-correct-but-wildly-impractical measurement system and the simple reality that everyone else just wants to say "a cup." It satirizes the kind of person who is so excited about cool physics concepts that they try to apply them everywhere, even when doing so is completely counterproductive.
The Landauer principle, formulated by Rolf Landauer in 1961, establishes that erasing one bit of information requires a minimum energy dissipation. Combining this with E=mc^2 allows you to express mass in terms of information -- a technically valid but absurdly circuitous way to measure flour.