Detailed Instructions: Given a scientific passage and an answer, generate a question for the given answer.
Problem:Passage: Helium has long been used in balloons and blimps. Since it is much less dense than air, it will float above the ground. We can buy small balloons filled with helium at stores, but large ones (such as the balloon seen above) are much more expensive and take up a lot more helium. Answer: helium
Solution:
What sort of gas is found in balloons and blimps, and is used to make them float in the sky?