Object permanence...the realisation that if you can't see 'it' it might actually still be there! Bring on the never ending game of peekaboo! My monster now doesn't go "AHHHHHHHHHH she's gone"! "Oh no, there she is", "AHHHHHHHHHH she's gone"!, repeat x10! Now (at nearly 6 months) every time the aptly named (by the husband) 'Mummy monster' lifts the fabric neatly placed over daughters head and says "peekaboo" there are fits of giggles and bucket fulls of dribble (from giggling too much).