A man's scrotum.
The balls, particularly when it's chilly and they're tucked up tight against the body. The foldy skin looks like the brains of a tiny baby.
"Ugh, I just caught a glimpse of your baby's brains...put those things away!"
When a man grabs his nut-sack, squeezes it from the base of his body, seperating the scrotum with a fist. The balls are in a "streched sack" state, which looks like the brains of a baby. Ball-hair accentuates this optical illution.
Hey look eveyone, baby's brains!!