"I don't want to monopolize the talk," Rolf said, but gave her a grin to show he wasn't being serious. "You can feel free to change the topic whenever you want."
Light-hearted as his comment might have been, it had the purpose of making things flow. He wanted to ask about the kiss, but felt it was probably too abrupt and awkward as a starting point. But what to start with...
"I take it from your comment that you'd like to search for the boundaries?"
There. An easy topic, and one where he could add the little information he knew about the discoveries the others had made.