Some wonderful people have made an interactive puzzle that wonderfully demonstrates how the Mercator Projection distorts size towards either pole. You start with 15 red countries scattered across a Mercator Projection of the world. A detail of the initial screen:
Move each red shape around the map until it coincides with the actual country; once "fitted" to the projection, the shape turns green:
Note that it's easier to do this with a computer, using a mouse, rather than on a tablet, with a finger or stylus. Also, it works only with the same 15 countries.