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"On the fly" means in haste and without advance preparation. "I don't have time to cook a big breakfast in the morning, so I just grab something on the fly that I can eat in the car."
"On the fence" means undecided, having no definite opinion.
"Have you decided whether you're going to study law or medicine when you go to college?"
"Not yet, I'm still on the fence."
on the fly 
Linear Mode
