Liz's reaction is so completely unexpected that Peter is completely and totally caught off-guard. As soon as his mind finally registers what's going on, his mouth naturally responds; a single arm curls around Liz's back, holding her.

And then he remembers that he shouldn't mess around with Liz while she's emotionally vulnerable. He breaks it off before it can become more than that, then squeezes her hand.

"That excited about science, huh."