The fruit of the osage orange tree (Maclura pomifera) litters the ground in Woods Creek Park.
An interesting tree, native to the United States. The tree, its wood, and the (inedible) fruit have numerous uses. (See Wikipedia.) Its main use around here is as a playground projectile and, most importantly, as a slightly creepy curiosity (especially when thrust toward your sister). The regional nickname, "monkey brains," aids in this effect.
Here's a close-up of the fruit: