Beer cans are curved at the top and bottom because curved walls are stronger, making them harder to crush. This is a fundamental engineering principle and is used to ensure firmer structural integrity in objects.
This principle is applied in other areas, such as automobile hoods and windshields. It is used in the construction of other objects and containers as well, such as barrels and oil drums. A firmer structural integrity is important in these types of containers because it helps to ensure that the container doesn't break easily and spill its contents onto the ground, a body of water or other surface, thereby wasting the product.