How long is a coastline? That depends. Do you measure around all those rocks jutting out from the beach? Do you measure the inside of a harbor? Do you take shortcuts when the coast is really jagged? And what about islands? It comes down to the "coastline paradox," in which the length of coastline can vary widely depending on how it's measured. The smaller units you use, the longer the coastline becomes.
But that's not the only problem in measuring a coast. The shoreline changes every day with the tides. It changes over time due to erosion, geologic activity, and human construction. It changes due to climate change. The problem, which you probably never thought about before, is explained in great detail in this video from RealLifeLore. The last minute is an ad. -via Laughing Squid