How did the theory of plate tectonics come to be a theory?

Plate tectonics is the idea that the earth's outer solid crust (the lithosphere) is fragmented into a few dozen "plates" that pass relative to each other across the earth's surface, like ice slabs on a lake.

A scientist named Alfred Wegener in 1915 proposed that the continents rammed through ocean basin crust, which would clarify why the shapes of many coastlines (such as South America and Africa) seem to match together like a puzzle.