This paper proves a theorem about the existence of an apparent horizon in general relativity, which applies equally well to vacuum configurations and matter configurations. The theorem uses the reciprocal of the surface-to-volume ratio of a region on a space slice to measure the radius of the region, and uses the minimum value Kmin of certain components of the extrinsic curvature to measure the strength of the gravitational field in the region. The theorem proves that, if the product of the radius times Kmin is larger than unity, then an apparent horizon must form, signaling the formation of a black hole.