Ohio Buckeye fruit

A friend brought me this buckeye. It had sat on her desk for about a week, and suddenly developed an indentation. Strangely enough, it looked like it was caused by someone's thumb pressing down on it (my own thumb fit in the indentation perfectly), but that's not what happened, since it was just sitting on her desk. It would be interesting to know the cause. Also, the fruits are poisonous to cattle and humans. Don't eat them.