Santa and Banta were studing geology. They went on a field trip studying rock formations in Russia. They arranged for a pilot to fly them into a remote area, to collect some specimens. While they're there, they collect six large rock specimens to take back to the lab, for further analysis.
The pilot returns to pick them up and they start loading everything into the plane. When the pilot sees the rocks, he objects strenuously.
"Those rocks are too heavy," says the pilot, "the plane can only take four, so you have to leave two behind."
Both argue with the pilot, telling him that the year before, they had collected six similarly heavy rocks and the pilot had allowed them to put all of them aboard. This plane is the exact same model and capacity.
Reluctantly, the pilot permits them to put the whole lot onboard. But when they attempt to take off and leave the valley, the little plane can't carry the load. They have to make an emergency landing in the wilderness.
Climbing out of the wreckage, Banta asks Santa, "Any idea where we are?"
"Yeah, I think so," replies Santa. "Must be about the same place where we had to make the emergency landing last year!"