Car-Boat
Explanation
The Joke
A man enthusiastically proposes the concept of a "car-boat" -- a vehicle that works as a car on land and then becomes a boat when you hit the ocean. His friend immediately points out, with calm logic, that there is no reason for such a thing: it is easier to just park your car and rent a boat, it would be cheaper, and even if car-boats were easy to make, you would still probably want them to be separate vehicles. The bottom panel cuts to a different pair discussing adulthood. One says "Being an adult must be a total nightmare," and the other replies, "It is like being dead but you are still there."
The Humor
The comic operates on two levels. The top half satirizes the recurring human impulse to combine two things that work perfectly well separately into one worse hybrid (car-boats, or more formally "amphibious vehicles," do exist and are generally impractical). The friend dismantles the idea with ruthless practicality, showing how childlike enthusiasm crumbles under adult reasoning. The bottom panel then pivots to comment on adulthood itself -- the grim joke being that the capacity for rational, joy-killing analysis is what defines being an adult. The juxtaposition suggests that the ability to immediately identify why something fun would not work is both the hallmark and the curse of maturity. The hyperbolic description of adulthood as being "like being dead but you are still there" underscores the bleakness of losing the capacity for impractical wonder.