If you tried to flatten a tennis ball, the sides would split and the shape would change. The same thing would happen if you tried to flatten a world map onto a piece of paper. It is impossible to create a two-dimensional map of the three-dimensional earth without stretching some places. Mapmakers call this stretching distortion. A globe can show size, shape, distance, and direction accurately, but since a flat map cannot be three-dimensional, we have to use map projections. A map projection is a way to show a drawing of the earth on a flat surface. All flat maps have distortion, so we use different map projections to meet different needs.
In 1569, Gerardus Mercator created a map where parallels and meridians cross at right angles. The Mercator Projection is excellent for navigation because it shows direction clearly. The Mercator Projection, however, has a great deal of distortion. In order to get the parallels and meridians to cross at right angles, Mercator stretched the areas further away from the poles and squeezed the areas closer to the equator.
An equal area map displays the shapes and sizes of things more accurately than a Mercator Projection. The Mollweide Projection is one of several equal area maps. The Mollweide Projection sacrifices accuracy of angle and shape in favor of accurate proportions in area. Compare Greenland and Africa on the two projections: Africa is actually fourteen times larger than Greenland, but on a Mercator Projection, the two regions are about the same size.
Many modern mapmakers use complicated mathematical formulas that combine the advantages of the Mercator Projection and equal area maps. The border of the western United States and Canada is the longest straight border in the world. Some map projections depict the border as a straight line; on other map projections you will notice a slight bend due to the curvature of the earth.