What is "curveballs"

Usage examples of "curveballs".

Castle started him off with a couple of curveballs, and he was patient enough to lay off them.

He was left-handed, threw nothing but curveballs, and had beaten us the year before, nine to two.

Screwing up his courage, John asked, "SusanSue speaking of curveballs, here's one for you.

As Brennan glanced through them he saw snippets about the sex life of politicians and the drug habits of bankers, notes on alliances between cops and gang figures, and even a list of which Dodgers had trouble with high fastballs and which were suckers for curveballs in the dirt.