Given a scientific passage and an answer, generate a question for the given answer.
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Question: Passage: Equinox comes halfway between the two solstices. At equinoxes, the Sun's rays shine most directly at the Equator ( Figure below ). The daylight and nighttime hours are exactly equal on an equinox. The autumnal, or fall, equinox happens on September 22 or 23. The vernal, or spring, equinox happens March 21 or 22 in the Northern Hemisphere. Answer: equinox

Answer: What event occurs between the two solstices?

Question: Passage: Alkenes are unsaturated hydrocarbons with at least one double bond between carbon atoms. Answer: alkenes

Answer: What are unsaturated hydrocarbons with at least one double bond between carbon atoms called?

Question: Passage: Another measure of the effectiveness of a machine is its mechanical advantage. Mechanical advantage is the number of times a machine multiplies the input force. It can be calculated with the equation:. Answer: mechanical advantage

Answer:
Another measure of the effectiveness of a machine is its what?