To understand how phone cases, windshields, and raincoats are all able to repel liquid, a good place to start is by examining nature—specifically, leafhoppers.
Chemistry Champion winner Mallory Hinks explains that superhydrophobic (roughly translated as "a whole lot of water-hating") coating imitates the ways of this fascinating bug. Like leafhoppers, when water droplets come in contact with it, the liquid forms a ball and sits on top until eventually rolling off.
This phenomenon can be explained by "contact angles": Water droplets have a contact angle with a leafhopper's wing of about 170 degrees—whereas most objects fall somewhere between zero and 120, resulting in the formation of a puddle. At 180 degrees, the droplet would form a perfect sphere.
To learn more about surface chemistry—and how it is possible to pour an entire bottle of red wine on your white shirt without leaving a stain—you can watch the full video from Reactions below: