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

Ex Input:
Passage: Figure 32.21 Supernovas are the source of elements heavier than iron. Energy released powers nucleosynthesis. Spectroscopic analysis of the ring of material ejected by Supernova 1987A observable in the southern hemisphere, shows evidence of heavy elements. The study of this supernova also provided indications that neutrinos might have mass. (credit: NASA, ESA, and P. Challis). Answer: supernovas

Ex Output:
What celestial events are the source of elements heavier than iron?

Ex Input:
Passage: Solutions can also become supersaturated , where the amount of solute dissolved exceeds its solubility. Supersaturation most commonly occurs when a saturated solution is slowly cooled. They occur frequently in geological and meteorological processes. Supersaturated systems are unstable, and eventually, the solute will precipitate until a saturated solution is regenerated. We can quantify supersaturation by looking at solubility curves. If the ratio of solute to solvent is above the saturation curve at the given temperature, the solution is supersaturated. If it is on the curve, the solution is saturated, and if it is below the curve, the solution is unsaturated. Answer: its solubility

Ex Output:
Solutions can also become supersaturated , where the amount of solute dissolved exceeds what?

Ex Input:
Passage: Earth and Moon orbit each other. This Earth-Moon system orbits the Sun in a regular path ( Figure below ). Gravity is the force of attraction between all objects. Gravity keeps the Earth and Moon in their orbits. Earth's gravity pulls the Moon toward Earth's center. Without gravity, the Moon would continue moving in a straight line off into space. Answer: gravity

Ex Output:
What keeps the earth orbiting the sun?