As an amateur astronomer, I deal with lenses a lot. Most astronomical lenses are complex, multi component things designed to reduce aberration (except gravitational lenses). The simplest lens is the convex lens, where the middle is thicker than the edges.
While modern astronomical lenses are made of thinks like crown glass, almost anything can become a convex lens provided it is transparent and can be curved.
Like this wineglass. The image is inverted because we are behind the focal point. Anyone else got any inages of "accidental lenses"