The sky's the limit." "It's a clean,renewable, highly efficient form of propulsion."

VOA: standard.2010.08.11

I think it was because people were overselling investments and saying, there's no limit to this investment but the blue sky above.

我想这是因为,当时人们过度宣传投资产品,并宣称,投资收益无止境,如头顶的蓝天那么遥远

耶鲁公开课 - 金融市场课程节选