Given a scientific passage and an answer, generate a question for the given answer.

Input: Consider Input: Passage: A lens that causes the light rays to bend away from its axis is called a diverging lens. Ray tracing is the technique of graphically determining the paths that light rays take. The image in which light rays from one point on the object actually cross at the location of the image and can be projected onto a screen, a piece of film, or the retina of an eye is called a real image. Thin lens equations are. Answer: diverging lens

Output: What do you call a lens that causes the light rays to bend away from its axis?

Input: Consider Input: Passage: An ionic bond is the force of attraction that holds together oppositely charged ions. Ionic bonds form crystals instead of molecules. Table salt contains ionic bonds. Answer: ionic bond

Output: What term means the force of attraction that holds together oppositely charged ions?

Input: Consider Input: Passage: Chordates include vertebrates and invertebrates that have a notochord. Chordates also have a post-anal tail, dorsal hollow nerve cord, and pharyngeal slits. Vertebrate chordates have a backbone, while invertebrate chordates do not. Invertebrate chordates include tunicates and lancelets; both are primitive marine organisms. Answer: backbone
Output: What do vertebrate chordates possess that invertebrate chordates do not?