Out of Sight, Out of Mind
Age: 4 Months Old
To play this game, you'll need something new and exciting that your child will want to touch (I often use a set of keys) and a baby blanket. If you show your 4-month-old baby a shiny set of keys, he'll probably want to reach out and grab them. But if you show him the keys and then hide them under the blanket, he'll stop looking for them. It's as if the keys suddenly ceased to exist.
This tells you that the sophisticated idea that objects are still around even when they're out of sight -- what psychologists call "object permanence" -- doesn't make sense to him yet. By the time he's about 10 months old, it will. If you put the attractive item under the blanket, a baby this age will simply reach under the barrier to find it. He gets it: Objects (and people) can be hidden and still exist!