A strange streak across the sky, a round object flying by, anything that you don't recognize could be aircraft for extra-terrestrials...
(See bottom of page for pictures)
Flat-bottom/Concave-Bottom Lenticular Disks
Coinlike Disk, with Variations
Bi-Convex Domed Disks
Spherical, Teardrop, and Multiple-Sphere
Angular: Cubes, Tetrahedras, Crescents, etc.
Elliptical: "Eggs", "Footballs"