What Do You Say?

What do you say when someone gives you information about someone else and the only thing you can think to reply is "OK"?

Ace got an e-mail from one of his cousins about her brother who is graduating from basic training soon. I would guess that congratulations is the proper thing to say, but only to the person graduating. Otherwise, it's kind of hard to tell what to say.

This is a slight problem because it comes up from time to time. I know what to say when someone dies ("I'm sorry for your loss."), when someone is pregnant, engaged or married ("Congratulations!"), and when someone is moving away ("I'll miss you."). I've even got stuff to say for other occasions that is situationally appropriate.

But what do you say when someone tells you news about something happening to someone else when you aren't sure how they feel? What do you say when someone blindsides you with something that doesn't have a particular "right" answer? What do you do when someone tells you that someone is graduating from basic training?

Someone, please tell me.