Use a tool like HandBrake to properly transcode the video if it won't open.
Use side-by-side comparisons of an elephant and a shrew to show how heart rates and metabolic needs vary by size. Size Matters 2.mkv.mp4
Which of these directions fits the specific video you have, or Use a tool like HandBrake to properly transcode
Create a "Quick Tech Fix" video or post about why you should never just type a new extension onto a filename. 3. Entertainment / Web Series Context Size Matters 2.mkv.mp4
Ask your audience: "If you were shrunk to the size of a marble, what everyday object would become your biggest obstacle?" 2. Tech Tip: Dealing with "Double Extensions"
If this is the BBC documentary hosted by Hannah Fry, the episode explores how physics changes when objects are scaled up or down.