Johnny Galecki and Kaley Cuoco Johnny Galecki and Kaley Cuoco

The Big Bang Theory's Penny and Leonard are about to take the next step: Being friends.

After several years of an on-again, off-again relationship, the pair will attempt to rekindle their friendship in Thursday's episode (8/7c, CBS), which sees them going on a non-date date. "They're defining that friendship," Johnny Galecki tells "They're discovering it and massaging it into something that it can be."

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Penny (Kaley Cuoco

) and Leonard have been on the outs since he first told her he loved her in Season 3, to which she replied "thank you." Things were only made worse when Raj's sister, Priya (Aarti Mann), came into the picture in Season 4, and Leonard rekindled an old flame with her, much to Penny's chagrin.Leonard's relationship with Priya caused a divide in their friendship, but with Priya out of the country, and their relationship on the rocks, Penny and Leonard will make a go of having a normal friendship. "It's taken a while to find that happy mutual ground for both of them, and there will be misunderstandings within it," he says.However, Galecki is quick to point out that a rekindled friendship does not mean they'll jump back into a relationship. "Not now," he says. "I do think there's a deep caring on both of their parts, but I think it would be premature at this point to take another stab at it. They have so much to learn. She's got a lot of baggage. He's got a lot of lessons to learn. I hope at some point down the road. I do think they may be the loves of each other's lives."

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Though Leonard has called himself single since he learned Priya slept with her ex-boyfriend — and Leonard kissed a fellow comic book enthusiast — Galecki hints that Priya may make a return appearance. "He's made mention that he's single, but at the same time, I've heard [executive producer] Bill Prady telling Aarti Mann to not worry, that this isn't the end of it, that she'll be back, but who knows? A lot of that changes in the writers' room."Do you think Penny and Leonard should end up together? Hit the comments with your thoughts.The Big Bang Theory airs Thursdays at 8/7c on CBS.