Babies in the Crib

There once were two babies both in the same room, with their cribs next to each other. After their mom tucked the babies in for sleep, one baby went to the other baby and said, “I can tell if you are a girl or a boy.”
The other baby said, “OK, what am I then?”
The baby went under the other babies blanket and came up a minute later, and said with a grin on his face, “You are a girl and I am a boy.”
The second baby asked, “And how do you know that?”
The baby replied, “I know because you have pink slippers and I have blue slippers.”