You're sitting next to a stranger who is crying.

[Iceglow]

You offer a tissue (and forshame if you haven't a tissue) and you say something like this:

"Is there something I can do to help you?"

They'll take it from there. Source: People come in to the office I work in crying a lot.

I don't carry tissues with me! D:

You're the worst Cobb, seriously! :P

Yeah generally offering a tissue and letting them take it from there is a good way to go. Normally when I offer a tissue I try and make a somewhat light-hearted comment that shows a little compassion and a willingness to talk but without obligation on their part.