eat-your-vegetables
Explanation
The Joke
In the first panel, a bald man with glasses (a classic SMBC scientist/dad archetype) presents a child with a plate of steamed cauliflower, cheerfully explaining that a substance has "solidified into the form of steamed cauliflower" and that the child had better eat it. This seems like a bizarre way to describe dinner.
The second panel, labeled "Earlier," reveals the context: the man had invented a machine that sucks a person's soul out of their body. The child reacts in shock ("What?!"). The implication is that the man used the soul-sucking machine on someone (possibly the child), and the extracted soul materialized into the form of steamed cauliflower -- which the man is now insisting the child eat.
The Humor
The joke works on multiple levels. First, it satirizes the classic parental demand to "eat your vegetables" by creating the most horrifying possible version of it -- the cauliflower is actually someone's soul. The man's breezy, matter-of-fact attitude about feeding soul-cauliflower to a child mirrors how parents often dismiss their children's objections to food. Second, there is something inherently funny about a soul taking the most unappetizing vegetable form possible. Of all the things a soul could solidify into, steamed cauliflower is perhaps the least inspiring, reinforcing the idea that the universe is fundamentally disappointing.