Mayim Bialik, Jim Parsons Mayim Bialik, Jim Parsons

Hewlett and Packard. Lady and Tramp. What do they have in common?

They're both couples costumes Sheldon (Jim Parsons) and Amy (Mayim Bialik) each want to dress as for Halloween on Thursday's The Big Bang Theory (8/7c, CBS) — except neither likes the other one.

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In our exclusive sneak peek below, the pair rack their brains — Venn Diagram-style, of course — to come up with a duo costume that they'd both like, but they aren't having much luck yet."I think it's a lost cause," Sheldon says."No, there are certain things that say to the world 'I have a boyfriend and he's not made up': matching costumes, hickeys and sex tapes," Amy retorts.What other costumes are on the table, er, in the diagram? Watch below to find out:What do you think Sheldon and Amy should dress up as?