The word fly has several meanings:

1. To move through the air using wings, like birds, butterflies, and bats do naturally. Humans learned to fly by inventing airplanes, helicopters, and other aircraft. When you fly somewhere on vacation, you travel by airplane. A flag flies when it waves in the wind at the top of a pole.
2. A small insect with two wings. The common housefly buzzes around kitchens looking for food. Fruit flies swarm around overripe bananas. Some flies, like horseflies, bite, while others just annoy us by landing on our food. Fishers use artificial flies (lures made to look like insects) to catch trout and other fish.
3. To move very quickly. When you're late for the school bus, you might fly down the stairs and out the door. Time flies when you're having fun, meaning it seems to pass quickly. A baseball can fly off the bat at over 100 miles per hour.
4. Something done or performed while moving through the air. A fly ball in baseball is one hit high into the air. Trapeze artists perform amazing tricks on the fly.

The phrase on the fly means doing something quickly while other things are happening, like a teacher adjusting lesson plans on the fly when students finish early.