Video shown in Developmental Psychology about Mary Ainsworth experiment on separation and stranger anxiety in infants.
Around 1:50, when the mother leaves the room and the baby's probably thinking "WHERE YOU GOING - WHY YOU SHUT DOOR IN MY FACE."
When mother returns, baby's reaction is "Lols okay, I'm sorry, please don't leave me."

The infant crying as above is seen as a normal, healthy reaction and a sign that the baby is securely attached to its caretaker (the mother). However, what experimenters look out for are how the baby reacts to the stranger and its environment with and without the mother's presence, and its response when the mother returns to it. These indications show whether the infant is securely attached, anxious-avoidant or anxious-resistant.

Interesting stuff. Might be observing babies next, haha.