WHO'S THERE?

An empty room you watch with care,

You want to know just who's in there;

Four walk in, and two walk out,

You look inside and all about.

There's only one person inside that place,

One happy, smiling, girlish face;

"Exactly right," I say to you,

"Only one person, certainly not two."

HINT:

You're not counting people.

Answer:

You're counting legs. Four legs walk in (two people). Two legs walk out (one person) - leaving two legs (one person) in the room.