Uncommon

After first contact, the initial focus was on scientific discussions, but after a while, they finally let some sociologists talk to each other.

Pretty quickly, it became apparent that there was very little in common between the planets, beyond very basic things like self-preservation and a concept of love/blornaz, although that one was shaky.

Undaunted, the two groups ran through everything they could think of, trying to find some commonality.

Eventually, out of desperation, they discussed mythologies, and got onto the topic of bogeymen.

This did have commonality, until humans were asked how they defeated their version of them.