analogy
Explanation
The Joke
The comic is titled "Thing I Hate: 'Down to Earth' explanations that don't add any clarity." A woman (who appears to be a teacher or science communicator) stands in front of a chalkboard and says: "The farthest known object in space is 13 billion lightyears away. If light years were grains of sand, that would be 13 BILLION grains of sand."
The joke is that the analogy is completely useless. The entire point of a "down to earth" analogy is to translate an incomprehensible quantity into something tangible and relatable. But this analogy simply replaces "lightyears" with "grains of sand" while keeping the same incomprehensibly large number (13 billion). You are no closer to understanding the distance than you were before, because 13 billion grains of sand is just as impossible to visualize as 13 billion lightyears.
The Humor
The humor comes from how perfectly the analogy mimics the form of a helpful explanation while providing zero additional understanding. It has all the hallmarks of a good analogy -- the relatable physical object (grains of sand), the confident delivery, the emphasized number -- but it is a tautology dressed up as pedagogy. It satirizes the common practice in science journalism and popular science of using analogies that sound enlightening but actually just restate the original fact with different nouns. The woman's earnest presentation style makes it even funnier, as she clearly believes she is making the concept more accessible.