cast
Explanation
This comic depicts two aliens sitting in a living room, looking at what appears to be a fireplace or chimney with a boxy object near it. One alien says: "You just pour some molten aluminum down the chimney and you get a perfect cast!"
The joke works by combining two meanings of the word "cast." In human culture, leaving out milk and cookies for Santa Claus on Christmas Eve is a beloved tradition. The aliens, however, have taken a very different approach to the chimney visitor: they've poured molten aluminum down the chimney to create a metal casting of whatever comes down it -- presumably Santa Claus. This is a reference to the real practice of pouring molten aluminum into ant hills or other cavities to create metal casts of the internal structure.
The humor is dark and absurd. The aliens treat the chimney intruder the way a pest control enthusiast might treat an ant colony, completely oblivious to (or unconcerned with) the fact that they've essentially killed Santa Claus by encasing him in molten metal. Their cheerful tone and domestic setting contrast sharply with the horrifying implication. The comic plays on the idea that aliens observing human customs might radically misinterpret them, or might simply have a very different set of values regarding uninvited visitors entering one's home through the chimney. Published on December 26th (the day after Christmas), the timing adds to the dark holiday humor.