Why do flys fly in zig zags?

Question

I've never seen em fly in a straightline

Answer 1

cuz they're stupid and drunk and can't decide where they are going.

Answer 2

Probably to make sure it's hard for predators (and you) to catch them.

Answer 3

Maybe the same reason a person fleeing from gunfire does; to make a more difficult target.

But the fly doesn't plan it, it's just programmed to fly that way all the time.