Peek-a-boo games are compelling for babies because infants in the sensorimotor stage lack:

Babies lack object permanence

Peekaboo demonstrates an infant's inability to understand object permanence.
Object permanence is understanding that the object continues to exist even though they don't see, hear, touch, smell it.
The very earliest understanding of object permanence emerges when the baby is 8–12 months.

If it doesn't have a nucleus its a prokaryote