A small water droplet on a computer monitor screen enables us to see the detail of a single pixel. Such droplets are almost perfectly spherical, ideal as optical lenses. We developed a novel fabrication method that uses water droplets as molds to make small, highly spherical lenses. Poly(dimethylsiloxane) (PDMS) spherical lenses have been made for demonstration: When uncured PDMS encapsulates water droplets and then solidifies, the cavities occupied by water become concave lenses or as a mold to make convex lenses. The light-collecting ability of these lenses is 6.25 times better than those made using traditional methods, while the cost is much lower. These lenses can be easily integrated with lab-on-a-chip devices for optical manipulation and/or detection.