First Harlow puts a newborn money in a cage with a surrogate dummy monkey doll.  The doll has fake fur and is warm and has milk bottles.  The monkey becomes very attached to this doll and relies on it for nutrition, and a comforting motherly body.  Next Harlow seperates the monkey into a larger cage with another doll as an option.

The next doll is just  a cold, wireframe structure although has milk and all the nutrients the monkey would need for survival.  The other doll is the one from before that was furry and it became attached to although this time it has no milk for it.

Watch what the monkey does: