A: When a small droplet of water is placed on a phone screen, it forms a curved semi-sphere and so acts as a convex lens. Because of its small curvature, this makeshift lens becomes quite powerful - so much so that it ends up magnifying all the tiny pixels of the phone screen. This magnification effect is achieved because the pixels are really close to the water droplet - enough that it is positioned at a distance less than its focal point (i.e. the point at which all light rays passing through the convex lens converge).
It is important to address that such a sharp magnification can only be achieved if the droplet is small, because only a lens with a small radius will curve “fast” enough, such that it can function as a good convex lens. But if the droplet becomes too large, then its surface tension will not be enough to compensate for its weight, and so it’ll simply turn flat.