Me:

I came to Earth to study humans, and I noticed a strange ritual involving groups of them standing inside a vertically moving metal box. They call it an elevator.

Once inside, they mostly avoid eye contact. Research suggests that making eye contact in an elevator causes an increase in human heart rate, which may eventually lead to death.

Isn’t that weird?

Adults:

The standard human model. They are pretty tall, though, especially by our standards. Perhaps the atmosphere at that elevation is more suitable for their respiratory systems. 

Kids:

Humans produce smaller versions of themselves that eventually grow up and replace them.

Perhaps this is necessary because human life science still hasn’t figured out eternal life.

Luggage:

Humans still haven’t invented space-compression technology, so the space inside their luggage is exactly as large as it appears from the outside.

They just in the way. What's even the point?

Couples:

Perhaps some form of conjoined twins.

They seem unable to separate from each other, even for a few seconds, and insist on staying together wherever they go.

Very difficult to deal with.