Her nipple.

Secure in the knowledge that your girlfriend is watching, you lean in toward Samantha and wrap your lips around her right nipple.

She gasps as you take her massive nipple into your mouth. You swirl your tongue around it, feeling it getting hard between your lips. It's big. It's the biggest nipple you've ever kissed. It's certainly the largest breast you've ever kissed.

And it feels so very strange to be interacting with a breast that doesn't belong to Katie. Strange, but good.

You hear Samantha's moans. And you feel her hands reaching out and beginning to paw at your hard crotch.

You pull back and look up at her. In all the time you've known her, you've never seen this expression on her face. This look of complete desire. It's telling you to keep going.

Which you do, leaning back down and sucking on her tremendous tit as her hand rubs your hard cock from outside your pants.

You caress your fingers down the side of her body again, feeling when you come to her hip, and changing your hands direction so it slides over her abdomen, over her soft bush, and curling two of your fingers into her pussy.

"Oh my god" you hear her whisper. She's working at your zipper now, but from her position, it's not the easiest thing to manipulate. So you reach down with your left hand and undo the button on your jeans, helping her grab and pull down the zipper, and tug the pants until they're gathered around your knees.

You curl a third finger inside of her wet slit as she pulls your raging hard cock from your boxers.

You wonder what Katie is making of this, over there, watching. Watching you and her very best friend. Your fingers inside her. Her hand now wrapped around your cock.

The feeling is amazing. The feeling of knowing she's watching. And you feel inspired to do things you might not normally do if you didn't know someone was looking.

You pull your hand from Samantha's crotch, and hold up your fingers to admire them, and hoping that Katie can see them too. The room isn't very light, but what light there is is glistening off them. They're simply soaked.

Sam bites her lips and let's out a little moan, "mmmm" that seems to suggest she's hopeful that you'll return your fingers to her soon. Still she jerks at your exposed cock.