Why Did She Care?

Sierra felt like she understood even less about Nathan Grant than she did before they hung out. Rock climbing was an unexpected activity choice considering she thought he was more of an indoor person. The only times she saw him out and about involved food, errands, or his sister. 

He was talented at it too! Watching him go on that difficult wall left her stunned. And that was after he saved someone's life and was completely casual about it. 

How did he know exactly when that man was going to fall to be able to catch him in time? She didn't buy his explanation about already watching him and having good reflexes. That sort of thing happened with him way too often, especially if he was the Amazing Skeleton Man. 

Who exactly was Nathan? What was his deal?