The London Bridge

Someone gets in a bridge and someone sits on top of them.

Why It's Impossible: These joint positions are completely unnatural for human beings. That, and we'd love to know the person who can hold a bridge with another human being sitting on their pelvis. That London Bridge ... is falling down. And we haven't even discussed how the penis and vagina would line up for penetration. Bloody ridiculous, as the Brits would say.