How To Keep the ball rolling?

She's already talking to you, dude, so she must trust you to a certain degree and suggests she's comfortable hanging out. I say listen to her and pick on a point she mentioned to steer some of the conversation to somewhere you really like. If she's yapping about, I dunno, how elephants are being used for labor, say something like... "On a related note, did you know that it takes some 3 days for an elephant's sperm to reach an egg? Crazy, isn't it?"

You must be well read though, like what the others have said, and know some cool triivia.

Just be natural when you divert the conversation. Since she was one talking about it initially, she's likely to forgive the diversion.

Make sure though that she's not just using you for a listening ear and only that.

You can also be proactive about it and already tell her you want to show her something... like bring her to somewhere with things that interest you or show something that interests you. You can then share what you know of that thing, ask her for her opinion about it. Showing excitement over something is exciting, so long as you don't get caught up in the subject that she feels you just wanted an audience.

Bring a book about massage. Practice with her.

That's the thing when you're meeting a girl during the day.

Well, for guys who are picking women up for sex, the topic doesn't usually matter. You have to learn to be sincere or fake it well though.

If sex is all you want when picking up women, you have to flirt a little more and gain more attraction. Good for you if the girl that you are hanging out with has the same mind set you have that time-sex. If both of you makes signals that you want to get laid then both of you will have a good night.