The shark, strangely enough, is one of the most popular fish for aquariums. The reason for this is that sharks adapt to their environment. If you catch a small shark and confine it, it will stay a size proportionate to the aquarium in which it lives.
Sharks can be six inches long and fully mature.
But turn them loose in the ocean and they grow to their normal size. I've noticed the same phenomenon with regard to followers of Christ. If we are challenged to live heroic lives for Christ, we have the capability. Our lesson for the day contains on of those growing, stretching experiences that came from time to time for Jesus' disciples. As usual Mark doesn't give us many details about what happened on the mountain.